gel coat

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

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.

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?"

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"

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

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

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

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.

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.

What’s the difference between P80 and 80 grit?