gel coat

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.

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

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

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

How much does crazing affect Polar?

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

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

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

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?

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"