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Old 04-21-2019, 04:37 AM   #1
rllull
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Cracking Paint

I am in the process of having my 79 painted. We are having an issued with the roof (between the T-top and rear window) with cracking. The cracks do not appear in the primer and have been showing up 24-36 hours after we apply the white paint. Any thoughts on possible causes or fixed. The area as been stripped and repainted several time know. Running out of thoughts.
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Old 04-21-2019, 06:03 AM   #2
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Originally Posted by rllull View Post
I am in the process of having my 79 painted. We are having an issued with the roof (between the T-top and rear window) with cracking. The cracks do not appear in the primer and have been showing up 24-36 hours after we apply the white paint. Any thoughts on possible causes or fixed. The area as been stripped and repainted several time know. Running out of thoughts.
I want to say there are unknown contaminants in the Fiberglass deep down leaching up to the surface.


May have to grind down a V groove 1/8" - 1/4" where the primer cracks are.
I would strip sand off the primer and clean with new Rags and lacquer thinner.
Soak a Rag and place over the surface for 1-2 hours.
Hopefully pulls out the contaminate.
Its usually silicone like Armor All.
Old oil will cause issues also.


Last few Corvettes I helped paint we stripped all the paint with Razor blades.
It took 2 days helping a bodyman friend.
He wanted no paint stripper used.
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Old 04-21-2019, 08:21 AM   #3
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It looks to me like there are cracks are in the glass. The expansion and contraction of hot and cold cracking the primer. A hairline crack can be easy to miss especially in a spot like this where you normally wouldn't expect them. You will likely need to repair the glass and of course refinish.

Tip: Probe the crack with the sharp tip of an xacto knife to determine how deep it actually is. If deeper than the primer
widen the crack a little bit with the knife before removing the primer. This will make it much easier to see once you've got primer off.

An old man's $0.02
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Old 04-21-2019, 06:52 PM   #4
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Originally Posted by 7TRoadster View Post
It looks to me like there are cracks are in the glass. The expansion and contraction of hot and cold cracking the primer. A hairline crack can be easy to miss especially in a spot like this where you normally wouldn't expect them. You will likely need to repair the glass and of course refinish.

Tip: Probe the crack with the sharp tip of an xacto knife to determine how deep it actually is. If deeper than the primer
widen the crack a little bit with the knife before removing the primer. This will make it much easier to see once you've got primer off.

An old man's $0.02
I have heard of stress cracks in C3 Glass. Never witnessed till now.
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Old 04-21-2019, 10:19 PM   #5
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Quote:
Originally Posted by Maniacmechanic1 View Post
I have heard of stress cracks in C3 Glass. Never witnessed till now.

I agree with 7TRoadster and Maniacmechanic1. The depth of the crack is anybody's guess at this point. Could be in the fiberglass itself or maybe on the surface. You didn't mention if the defects show up in the exact same place each time, but I'm assuming that's the case.



Just my opinion, but in my experience if a defect shows up that quickly then it's almost certain to be some kind of reaction between the paint and some unknown chemical residing in what is most likely a surface crack or scratch. Remember that primer can be slightly porous, that's what gives the paint something to bite into.


I have done jobs on metal body cars and on a couple of them I've had defects quickly materialize that look remarkably like your picture. Both cases turned out to be incompatibility between the paint that I sprayed over existing topcoat. It looked exactly like a crack, but had to be a virtually invisible scratch in the old topcoat or primer.


Back when the Beatles were popular and I had hair, I quickly learned that refinishing using lacquer over the then-common acrylic enamels was a good way to use up a lot of sandpaper. Everywhere there was a sand scratch the lacquer chemicals dug in and caused either a ridge or canyon to develop. End result looked exactly like a crack.


It would help us to know if you are going over original factory paint or a repaint from years past, what kind it was, what you are now using for primer/sealer, final finish, prep technique, etc. One thing is for sure, don't keep doing the same thing while expecting different results. Something is clearly wrong and you need to find out what it is.


To be sure you should probably remove all previous paint, right down to the body. Don't sand it like you would a metal panel. It can be done, but one slip-up will create a bunch of work to fix. I'm currently using the stripper method recommended in 7TRoadster's sticky. A little slow but you will wind up with a body free of sanding induced damage.



Good luck, don't hesitate to ask questions here!
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