gel coat

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

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'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

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

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

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

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

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.