gel coat

[BobWa43]

Is gel coat crazing purely a cosmetic problem or does it pose a threat to the structural integity of the underlying fiberglass? I realize this is a controbersial issue but I would really like to hear the current opinions of members with experience refinishing gel coated gliders.

[[email protected]]

I ran a glider repair station for 20 years and have ground out many a crack.............never found any of them that extended into the underlining structure. I’m talking about cracks like those coming from spoiler box, a zig-zag looking crack is most likely indicating underlying structure has moved and must be ground out so as to see the structure below. You can get other opinions, in fact the Australian FAA, published an AD saying all cracks are structural and make the bird unairworthy!
Let the games begin............
JJ

[2G]

On Monday, September 23, 2019 at 11:13:25 AM UTC-7, wrote:
> I ran a glider repair station for 20 years and have ground out many a crack............never found any of them that extended into the underlining structure. I’m talking about cracks like those coming from spoiler box, a zig-zag looking crack is most likely indicating underlying structure has moved and must be ground out so as to see the structure below. You can get other opinions, in fact the Australian FAA, published an AD saying all cracks are structural and make the bird unairworthy!
> Let the games begin............
> JJ

That makes sense as gel coat is a coating on top of the structural fibers. Crazing looks horrible, but the structure is fine.

Tom

[Eric Munk]

Found two gliders during refinish where age-related gelcoat cracks
had gone into the structure. One required local repairs only, the
other a complete replacement of inner 4 metres of top wing skin
glassfibre (outer layer). The gelcoat cracks on that were
incredible.

Localised gelcoat cracks found some to be overload/accident
damage related (pushed in during belly landing eg). Others
radiatong cracks from airbrake box corners and the like.

Most major manufacturers have published service bulletins that
are a great help.

Eric


At 03:31 24 September 2019, 2G wrote:
>On Monday, September 23, 2019 at 11:13:25 AM UTC-7,

>wr=
>ote:
>> I ran a glider repair station for 20 years and have ground out
many a
>cra=
>ck............never found any of them that extended into the
underlining
>st=
>ructure. I=E2=80=99m talking about cracks like those coming
from spoiler
>bo=
>x, a zig-zag looking crack is most likely indicating underlying
structure
>h=
>as moved and must be ground out so as to see the structure
below. You can
>g=
>et other opinions, in fact the Australian FAA, published an AD
saying all
>c=
>racks are structural and make the bird unairworthy!
>> Let the games begin............
>> JJ
>
>That makes sense as gel coat is a coating on top of the structural
fibers.
>=
>Crazing looks horrible, but the structure is fine.
>
>Tom
>

[son_of_flubber]

How much does crazing affect Polar?

[Tango Eight]

On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
> How much does crazing affect Polar?

Not at all (provided that the surface is being maintained).

As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure.

T8

[[email protected]]

On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote:
> On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
> > How much does crazing affect Polar?
>
> Not at all (provided that the surface is being maintained).
>
> As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure..
>
> T8

I have seen plenty of cases of imprinting on the top layer of the structural laminate under finish cracks. None have progressed beyond the top layer of the laminate. Some of those areas appeared to be compromised to a degree.
These areas are also a path to moisture which can degrade the laminate.
In very thin structures, like control surface trailing edges, the gelcoat does add stiffness. Finish cracks, usually just forward of the trailing edge joint are common on some types of ships. Move away from the crack and the surface is notably stiffer. Sand the gelcoat off and there usually is no underlying structural failure.
Crazing is an indication that future cracking is coming. Crazing is mostly cosmetic. Cracking is not in my experience.
FWIW
UH

[son_of_flubber]

> On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
> > How much does crazing affect Polar?
>
On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote:
> Not at all (provided that the surface is being maintained).

What does 'the surface is being maintained' mean? Can you 'buff out' crazing to the point that it does not increase drag?

I'm surprised that crazing towards the leading edge does not affect laminar flow. How is crazing different than 'bug factor'?

Does crazing progress to a point where it affects drag? Is it still called crazing when that happens?

[Tango Eight]

On Tuesday, September 24, 2019 at 12:09:07 PM UTC-4, son_of_flubber wrote:
> > On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
> > > How much does crazing affect Polar?
> >
> On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote:
> > Not at all (provided that the surface is being maintained).
>
> What does 'the surface is being maintained' mean? Can you 'buff out' crazing to the point that it does not increase drag?
>
> I'm surprised that crazing towards the leading edge does not affect laminar flow. How is crazing different than 'bug factor'?
>
> Does crazing progress to a point where it affects drag? Is it still called crazing when that happens?

Proper maintenance means sanding (we're talking 800 grit and finer here, with a spline board or sanding bar for support) followed by polishing with either white rouge (aka "hard wax") or a very high end auto body polish (e.g. 3M Perfect It), followed by wax. That treatment renders any crazing a non-factor aerodynamically. As crazing progresses, you'll find that you need to repeat this treatment periodically. Your fingers will tell you when. If the finish feels smooth, aerodynamically, it is smooth.

If/when the finish starts shedding chunks of material, flaking or cracking up in a way that cannot be levelled with sanding and polishing, well, that's a different story, and patching or all out refinishing will be required to re-obtain full performance. Some readers will be familiar with a certain rather sketchy looking glider that had been patched (extensively) with spackle... and the hell of it is, that glider went pretty darned well!

T8

[[email protected]]

The guy in the cockpit makes up for the craising on the wings.
A couple of wrong decisions by the guy flying the super true laminar perfect winged machine, and a few right decisions by the bondo wing flying guy and they'l be tied lol.

[[email protected]]

On Tuesday, September 24, 2019 at 1:35:38 PM UTC-4, wrote:
> The guy in the cockpit makes up for the craising on the wings.
> A couple of wrong decisions by the guy flying the super true laminar perfect winged machine, and a few right decisions by the bondo wing flying guy and they'l be tied lol.

True- BUT- All the points made by good glider preparation are points that don't have to be given back.
From a guy who had two 999 point days this year. One of those made the difference between silver and bronze in nationals.
UH

[[email protected]]

Yes, agreed. I just finished working up an article regarding 5 principles necessary to win at low performance racing. The first principle is all about preparation.

But one of the other (and imop) more important factors is making the proper inflight decisions necessary to fly further or faster.

You know that the presence of a single point difference between 1st and 2nd place amounts to about a single false-thermal attempt, or maybe a few degrees of heading difference along an energy line. Its the "Moffat" principle revisited; namely "win by not loosing".

[Papa3[_2_]]

To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level.. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.

p3





On Tuesday, September 24, 2019 at 10:49:24 AM UTC-4, wrote:
> On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote:
> > On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
> > > How much does crazing affect Polar?
> >
> > Not at all (provided that the surface is being maintained).
> >
> > As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure.
> >
> > T8
>
> I have seen plenty of cases of imprinting on the top layer of the structural laminate under finish cracks. None have progressed beyond the top layer of the laminate. Some of those areas appeared to be compromised to a degree.
> These areas are also a path to moisture which can degrade the laminate.
> In very thin structures, like control surface trailing edges, the gelcoat does add stiffness. Finish cracks, usually just forward of the trailing edge joint are common on some types of ships. Move away from the crack and the surface is notably stiffer. Sand the gelcoat off and there usually is no underlying structural failure.
> Crazing is an indication that future cracking is coming. Crazing is mostly cosmetic. Cracking is not in my experience.
> FWIW
> UH

[Charles Longley]

Bit of thread drift but it has to do with gel coat. I am currently doing a life extension on a Libelle (OD). It’s original gel coat is in far better shape then any of the ASW 20’s I looked at including the one I bought. It’s one of the earlier Libelle’s so 12 plus years older then a 20. I am not sure of why the difference? Did Glasflugel have better quality control than Schleicher? Or was it a different material?

[[email protected]]

Glasflügel used Lesonal Schwabbellack instead of Vorgelat Gelcoat. They also did not paint the seams with a quick drying mixture, including too much hardener.

[BobWa43]

On Monday, September 23, 2019 at 11:50:30 AM UTC-4, BobWa43 wrote:
> Is gel coat crazing purely a cosmetic problem or does it pose a threat to the structural integrity of the underlying fiberglass? I realize this is a controversial issue but I would really like to hear the current opinions of members with experience refinishing gel coated gliders.

Many thanks to all of you who commented. It was a most enlightening discussion.

[[email protected]]

> True- BUT- All the points made by good glider preparation are points that don't have to be given back.
> From a guy who had two 999 point days this year. One of those made the difference between silver and bronze in nationals.
> UH

Ironically, UH oversaw/assisted/provided the skilled work (vs. the grunt sanding work) on the glider that was the 1 point beneficiary of a complete refinish job.

No good deed goes unpunished. :)

JB

[[email protected]]

On Wednesday, September 25, 2019 at 6:54:50 PM UTC-4, wrote:
> > True- BUT- All the points made by good glider preparation are points that don't have to be given back.
> > From a guy who had two 999 point days this year. One of those made the difference between silver and bronze in nationals.
> > UH
>
> Ironically, UH oversaw/assisted/provided the skilled work (vs. the grunt sanding work) on the glider that was the 1 point beneficiary of a complete refinish job.
>
> No good deed goes unpunished. :)
>
> JB

The reward was seeing one of my best friends in soaring get his best ever nationals finish.
UH

[Tango Whisky]

Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*:
> To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.
>
> p3

These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).

These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.

Bert
Ventus cM "TW"

[[email protected]]

On Thursday, September 26, 2019 at 8:59:23 AM UTC-4, wrote:
> On Wednesday, September 25, 2019 at 6:54:50 PM UTC-4, wrote:
> > > True- BUT- All the points made by good glider preparation are points that don't have to be given back.
> > > From a guy who had two 999 point days this year. One of those made the difference between silver and bronze in nationals.
> > > UH
> >
> > Ironically, UH oversaw/assisted/provided the skilled work (vs. the grunt sanding work) on the glider that was the 1 point beneficiary of a complete refinish job.
> >
> > No good deed goes unpunished. :)
> >
> > JB
>
> The reward was seeing one of my best friends in soaring get his best ever nationals finish.
> UH

Thanks, Hank! It was one of the high points of my 50+ years in soaring. Just to reinforce your message, on the last day I made a 20 mile desperation final glide thru mostly sinking air and took a small penalty because I was slightly below the floor of the finish cylinder. IIRC, if I had been 3 feet lower, I would have lost that one point. Sure, there were all kinds of things I could have done earlier in the flight to more than make up for that. But I didn't. At the end of the flight, it came down to one tenth of one percent on that glide. That was my reward for all those hours spent with you pulling templates off my ASW 24 wing, removing the old gel coat carefully, and then building the profile back up the right way. Thanks, again, for your care and attention (and patience!) in that effort.

The above situation is rare. More typically, having that small extra percentage makes it possible to stay with the fleet moving down a fast leg on a blue day vs. slowly being dropped. It's probably not worth worrying about it unless you're flying competitively but it is for me. We're all different. Fortunately, soaring accommodates all of us.

Chip Bearden
JB

[[email protected]]

On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
> Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*:
> > To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.
> >
> > p3
>
> These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).
>
> These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.
>
> Bert
> Ventus cM "TW"

If the structure is not compromised we fill the "million pinholes" using polyester surface filler applied with a foam roller. This works filler into the voids and avoids the bridging that results if filler is sprayed.
Laminating another layer, without peeling the top ply,can result in undesired shape change, and when sanded smooth, can leave defects that still require detail filling.
FWIW
UH

[Papa3[_2_]]

Actually no - these are not the normal voids. Being a sergeant in the "pinhole patrol army", I'm very familiar with these voids.

The pitting I'm talking about is 100%, directly related to the failure of the gelcoat. The crazing lines literally transfer down into the outermost layer of the substrate. When you put a 6x loupe on the crazing lines after the gelcoat is sanded off, you see that there is a) slight discoloration and b) tiny chunks of epoxy coming out of the glass matrix.

I have pictures, but not under magnification. Trust me - the key guy who looked at this is a materials scientist who works for one of the top electron microscopy vendors specializing in materials failure analysis.

Cheers,
P3



On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
> Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*:
> > To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.
> >
> > p3
>
> These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).
>
> These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.
>
> Bert
> Ventus cM "TW"

[Charles Longley]

How many wing failures have composite gliders had over the years?

[Roy B.]

On Monday, September 23, 2019 at 11:50:30 AM UTC-4, BobWa43 wrote:
> Is gel coat crazing purely a cosmetic problem or does it pose a threat to the structural integrity of the underlying fiberglass?.

About 28 years ago I refinished a glider (an ASW-17) with Simtec Prestec - which was then a somewhat new product for glider refinishing ( it was originally sold as a coating for radomes). The glider is now in Canada. Can anybody tell me today how that product has held up over time (compared to the original Vorgetlat and Schwabelack)?

ROY

[Bill G]

Excellent Question

[[email protected]]

I have a 1988 AS-W20 that had modifications done (tailwheel fairing, Canopy refit) that required recoating with Prestec. They are essentially perfect today. No crazing or yellowing.

[[email protected]]

On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
> How many wing failures have composite gliders had over the years?

Exactly.

[Papa3[_2_]]

On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
> How many wing failures have composite gliders had over the years?

A fair number, but I can't think of any that were due to normal flight. For example, at least 1 or 2 Slingsby Vegas came apart, but I believe both of those were due to exceeding design limits. Same for a Zuni. A fair number of others came apart due to flutter.

P3

[[email protected]]

On Friday, September 27, 2019 at 7:16:17 PM UTC-4, Papa3 wrote:
> On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
> > How many wing failures have composite gliders had over the years?
>
> A fair number, but I can't think of any that were due to normal flight. For example, at least 1 or 2 Slingsby Vegas came apart, but I believe both of those were due to exceeding design limits. Same for a Zuni. A fair number of others came apart due to flutter.
>
> P3

A control surface that has been compromised(easy to do given light structure) could lead to flutter due to reduced stiffness and you could lose a ship..
Glider structures are over built in some places for durability, but not everywhere.
UH

[Charlie Quebec]

A very good AN on this subject, including expert studies showing propagation of cracks into epoxy layer. Worth a look.
http://www.doc.glidingaustralia.org/index.php?option=com_docman&view=download&alias=1093-gfa-069an&category_slug=an-051-100&Itemid=133

[[email protected]]

On Friday, September 27, 2019 at 4:16:17 PM UTC-7, Papa3 wrote:
> On Thursday, September 26, 2019 at 8:40:34 PM UTC-4, Charles Longley wrote:
> > How many wing failures have composite gliders had over the years?
>
> A fair number, but I can't think of any that were due to normal flight. For example, at least 1 or 2 Slingsby Vegas came apart, but I believe both of those were due to exceeding design limits. Same for a Zuni. A fair number of others came apart due to flutter.
>
> P3

P3 - I was the owner of one of the Vegas that crashed (N4312B, in 1987 in Hemet) - the root cause was elevator flutter resulting in failure of the elevator linkage. No overspeed, no structural failure, just flutter followed by loss of pitch control causing the pilot to bail out (unfortunately at low altitude, with serious injuries).

Cheers,

Barry

[George Underhill]

Charlie Quebec's AN posting is very informative. Thanks for posting.

[Nick Kennedy[_3_]]

Crazing Gelcoat is just like a wife who drinks too much.
You've got to get rid of it and its going to cost alot of money.

[Charlie Quebec]

The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

[Roy B.]

Crazing Gelcoat is just like a wife who drinks too much.
You've got to get rid of it and its going to cost alot of money.


Nick:
More like a stalking ex-girlfriend. If you don't do it right it's going to come back.
ROY

[[email protected]]

On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
> The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
....
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

[[email protected]]

On Monday, October 21, 2019 at 3:37:59 PM UTC-4, wrote:
> On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
> > The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.
>
> The AN is an interesting read--and an alarming one given the warning that:
>
> "Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
> ...
> (4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
> (5) Transfer into and through the foam layers of sandwich skins.
> (6) Transfer from skin layers into spar caps."
>
> However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:
>
> "It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"
>
> That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?
>
> Chip Bearden
> JB

I can see no basis to an expectation that a crack propagating into an outer skin would continue through a core foam and then start again through inner skin.
I've seen imprinting of gelcoat cracks in the outer layer of skin laminate, but have not seen one that progressed beyond the outer(usually 110) layer.
The real message is that if the glider is in gelcoat failure(deep cracking, cupping, peeling) real attention needs to be paid.

FWIW
UH

[[email protected]]

I concur with Uncle Hank’s assessment. After grinding off all gelcoat, I have seen what looks like a ghost of the crack that was there, but close examination with strong light and magnification, showed some resin degradation in the outside layer, but I could not see broken fibers. That said, I believe deep cracks should be ground out, filled, contoured and re-finished.
Refinishing the whole ship is the hardest work I have ever done, my hat’s off to anyone willing to take on that nasty job!
JJ

[Charlie Quebec]

I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

[2G]

On Wednesday, October 23, 2019 at 1:45:51 PM UTC-7, Charlie Quebec wrote:
> I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

What, exactly, is "dry penetration testing?"

[[email protected]]

On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
> I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

I have never had to replace glass due to crack propagation. I have had to replace glass due to damage during finish removal countless times. Sometimes the top layer is in a condition that is makes sense to peel the first layer and replace it.
UH

[James Thomson[_2_]]

At 03:05 24 October 2019, 2G wrote:
>On Wednesday, October 23, 2019 at 1:45:51 PM UTC-7, Charlie Quebec
wrote:
>> I know of several cases where the top layers of glass had to be
replaced
>due to crack propagation. This was only confirmed by dry penetration
>testing.
>
>What, exactly, is "dry penetration testing?"
>
I think that it is Aussie code for "dye penetrant"

[Tango Eight]

On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
> I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

I'd like to see those results.

T8

[Papa3[_2_]]

On Thursday, October 24, 2019 at 10:15:44 AM UTC-4, Tango Eight wrote:
> On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
> > I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.
>
> I'd like to see those results.
>
> T8

That makes 2 of us. The reason may (may) be because it's just easier to achieve a smooth finish by peeling off the top layer and replacing rather than structural concerns.

Back in the dark ages when I was in school (30 years ago), the materials science lab had an ongoing experiment related to aging of gelcoated fiberglass. At that time (around the same time as the Aussie report), I believe they had yet to see any significant degradation of structural properties even with significant failures of the gelcoat. In fact, I believe (can't recall exactly) that the test surface was taken from a wrecked glider!) I'll see if I can dig up the professor's name and see if there was any output published.

p3

[Roy B.]

I'd like to hear from the professionals about the best way to remove old gel coat without damaging the underlying glass. There are a lot of suggestions out there (air file, very coarse random orbital sander, etc. etc.) and when I did it (once and for the last time, I swear) I used a fairly coarse hand held belt sander.
What do you guys use for this job? On flat surfaces? On compound curves?
ROY

[Richard Pfiffner[_2_]]

On Friday, October 25, 2019 at 12:11:42 PM UTC-7, Roy B. wrote:
> I'd like to hear from the professionals about the best way to remove old gel coat without damaging the underlying glass. There are a lot of suggestions out there (air file, very coarse random orbital sander, etc. etc.) and when I did it (once and for the last time, I swear) I used a fairly coarse hand held belt sander.
> What do you guys use for this job? On flat surfaces? On compound curves?
> ROY

Roy

Process I used.

http://www.craggyaero.com/ventus.htm

Richard
www.craggyaero.com

[Papa3[_2_]]

The picture about midway down the page on Richard's blog pretty much mirrors the way we do. Same tools. Same papers. Same hand-sanding blocks. I've actually been creation a Bill Of Materials on Amazon that has most of our stuff. Maybe I'll get around to cleaning it up over the weekend. It's at least good as a starting point...

Erik Mann
ACA Chief Refinishing Officer (CRO) :-)
P3

On Friday, October 25, 2019 at 3:25:29 PM UTC-4, Richard Pfiffner wrote:
> On Friday, October 25, 2019 at 12:11:42 PM UTC-7, Roy B. wrote:
> > I'd like to hear from the professionals about the best way to remove old gel coat without damaging the underlying glass. There are a lot of suggestions out there (air file, very coarse random orbital sander, etc. etc.) and when I did it (once and for the last time, I swear) I used a fairly coarse hand held belt sander.
> > What do you guys use for this job? On flat surfaces? On compound curves?
> > ROY
>
> Roy
>
> Process I used.
>
> http://www.craggyaero.com/ventus.htm
>
> Richard
> www.craggyaero.com

[Richard Pfiffner[_2_]]

On Friday, October 25, 2019 at 12:52:35 PM UTC-7, Papa3 wrote:
> The picture about midway down the page on Richard's blog pretty much mirrors the way we do. Same tools. Same papers. Same hand-sanding blocks. I've actually been creation a Bill Of Materials on Amazon that has most of our stuff. Maybe I'll get around to cleaning it up over the weekend. It's at least good as a starting point...
>
> Erik Mann
> ACA Chief Refinishing Officer (CRO) :-)
> P3
>
> On Friday, October 25, 2019 at 3:25:29 PM UTC-4, Richard Pfiffner wrote:
> > On Friday, October 25, 2019 at 12:11:42 PM UTC-7, Roy B. wrote:
> > > I'd like to hear from the professionals about the best way to remove old gel coat without damaging the underlying glass. There are a lot of suggestions out there (air file, very coarse random orbital sander, etc. etc.) and when I did it (once and for the last time, I swear) I used a fairly coarse hand held belt sander.
> > > What do you guys use for this job? On flat surfaces? On compound curves?
> > > ROY
> >
> > Roy
> >
> > Process I used.
> >
> > http://www.craggyaero.com/ventus.htm
> >
> > Richard
> > www.craggyaero.com

Don't you mean chief therapy sander!

Richard

[Charlie Quebec]

Oops, dye penetrant testing, damn auto carrot. if you didn’t do due test, you don’t really know if cracks went further or not.
The cracking had only effected the epoxy matrix, but the glass had to be removed to fix them
From Schlesinger tech note:

As all the outside skin of the aircraft is dimensioned for stiffness, there are no critical mechanical strength problems, even if some cracks have gone down into the fiber composite structure and have already attacked the resin matrix base.
The unknown ageing effects caused by the influence of moisture and UV on the unprotected fiber composite structure are more dangerous.

[Per Carlin]

I recommend to use Mirka Autonet or similar product for drysandning.
I have a Mirka Deros 650 orbital sander connected to a industrial vacuumcleaner. 99,9% of all the dust are collected and non the **** are stuck in my nose.

I remove the original celcote with P80 (not 80-grit), then P120, P180, P240 and P400 prior to the primer. Sandning the primer with P180, P240 and P400 prior the topcote.
If you go directly from P80 to primer is there are big risk that the grinding scratches will shine through the primer and the topcote after a season of two.

[Roy B.]

Per
Do you use the same machine for sanding the primer that you use to remove the gel coat?
ROY

[[email protected]]

On Saturday, October 26, 2019 at 5:39:05 PM UTC-4, Roy B. wrote:
> Per
> Do you use the same machine for sanding the primer that you use to remove the gel coat?
> ROY

We use Porter Cable orbital sander with Mirka 23-388-080 adhesive backed discs for most of the removal. Other grits are sometimes useful. Last work is by hand.
Primer/filler is hand shaped. Final finish sand is orbital with finer grit.
Contact directly if you want more info.
UH

[Per Carlin]

I use the same machine for removing gelcote, grinding primer and polishing/waxing both new and old paint. It was an investment of about 500€, i use the machine for all kinds of other grinding jobs than on my glider (furnature, etc) so I consider it as an good investment.

[Charles Longley]

What’s the difference between P80 and 80 grit?

[Dave Nadler]

On Friday, November 1, 2019 at 3:02:17 PM UTC-4, Charles Longley wrote:
> What’s the difference between P80 and 80 grit?

https://www.grainger.com/content/supplylink-sandpaper-guide