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

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?

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

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.