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Old 07-19-2009, 11:11 PM
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Question about paint cracking on fiberglass

Hello guys. Been reading here for quite a while, but this is my first post.

I've got a 69 Corvette I've had since 71. By the mid 70's it had some alligator type cracking in the original paint on the passenger side door about 4 inches down from the top.

I repainted it in the late 70's. Sanded the original paint down to the factory prime coat, except on that door where I took it to bare glass. Couldn't really see anything to cause the cracking, but the surface of the glass in that area did look a little "different", like maybe there was more fiber and less resin near the surface. Looked different, but really couldn't feel anything with the bare hand.

Primed and blocked it using lacquer primer surfacer and repainted it using Dupont lacquer.

Paint held up fine, with the exception of the same area on the door. That started to show some cracking after 5 or so years, to about the same degree as the original.

Painted it again in the last half of the 80's. This time, I took everything to bare glass. I'd done some fender flares prior to the first repaint to accomodate wide tires. Tired of that, so I cut off the fenders and quarters at the bond strips and replaced all 4 with factory parts. Shot everything with Feather-fill for blocking, followed by lacquer base primer, and another lacquer topcoat job.

The same door developed the same cracks again in a few years, even though the car was seldom driven and stayed in one of my garages most all the time. Today, with 20+ years on the paint, everything's still good with the exception of that area on the door. The fender and quarter seams are still invisible, and there's no other cracking anywhere else on the body.

I'd like to redo the car again. Been concentrating mostly on my business for the last 20 yrs or so, and I need a project now to get me off the couch in the evenings. But, being the smart (not so young anymore) guy I am, I'm pretty sure I need to do something about the area on the door, and that's the purpose of this tome.

I've thought about grinding it down somewhat and refilling with glass reinforced bondo, but not sure how well that would work. Because the Vette panels are molded from sheet material, I don't think they're thick enough to allow cutting the area down enough to reglass it with some thin mat without possibly causing some general weakness in the door, and the possibility of more paint cracks, just from a different source. Really wondered if I oughta just try to scrounge up another door. Haven't checked to see if the skin is still available from GM, but I sorta doubt it. In any case, I'd rather fix what's there than replace, as long as its a reliable solution.

Anyway, that's about the whole history of the thing, and any suggestions would be much appreciated. I do plan to go back with BC/CC this time, and not lacquer.

I didn't intend for this to be quite so long, but I'm a mod on another non car related site and I hate it when someone asks a question without providing basic details anyone capable of giving an answer is going to want to know. Thanks for taking the time to read

Cliff

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Old 07-20-2009, 08:03 AM
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Part of your problem is lacquer, it is not durable and will craze (crack) over time if too much clear has been applied. The answer to your problem is a quality BC/CC. The first thing you need to do after removing all the lacquer products is to seal the surface with a quality epoxy primer. You can spend tons on a high profile epoxy primer like PPG or DuPont and not get the quality of SPI for lots less money. Once you have several coats of SPI epoxy on the body you can then apply some blocking primer. There is a member on this board who has extensive knowledge of body work on Vette's, his usersname is Shine....PM him or wait for him to reply to this thread.

Vince
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Old 07-20-2009, 09:03 AM
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Thanks Vince. I'd agree with your comments in general regarding lacquer, but I'm pretty sure this is some underlying problem with the glass itself since GM's original lacquer job failed in the same time frame and same confined area as both of mine have. There's no clear on the car, and not a sign of stress cracking anywhere else on the paint.

Assuming the problem is under the paint, and not the paint itself, I'd be hesitant to try anything paint-wise without first doing something to the door, whatever that something may be. Its sorta like if you were to paint some structural fabrication and a weld cracked later on. Regardless of what paint or primer is on top, the crack is gonna come thru. I'm pretty sure this is happening from the panel surface outward, particularly since it came right thru the feather-fill as well as the primer and paint.

I do plan to take everything to bare glass, and start from there with epoxy this time. Last time I painted the car, the BC/CC paints were just getting popular and the carmakers were having paint failures left and right, so I decided not to experiment in that direction. No doubt today BC/CC is the only way to go on any vehicle that's worth the effort of repainting.

Cliff
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Old 07-20-2009, 09:56 AM
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redo the car with epoxy . forget polyester as it will allow the print to telegraph through . what happened is the lacquer checked and the glass "took" the print. fiberglass is after all polyester. if the area has any damage then sand it down good an put one layer of veil cloth over it.
i use only epoxy on corvettes . no polyester and absolutely no junk urethane primer. good luck with it.
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Old 07-20-2009, 11:11 AM
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I'm not sure that I would concur with taking it down to bare glass again. To quote one of my knowledgable friends "nothin sticks to paint like paint". There are times when removing the paint is required, it is useful when you have metal and need to weld or de-rustify, or with fiberglass when you need to make a repair, or when layers of paint are too thick to work with. Unless the paint is lifting, you can rely on the paint that was applied in the 80s to be good enough to work with. You will need to remove the paint on the door that has the problem.

Frequently the mako bodies had paint problems from the factory. Your problem is not that uncommon, because the gel coat (epoxy) layer was not properly applied, or was removed during fit and finish before painting, the paint will not adhere properly to the polyester (fiberglass).

You will need to work the door down, apply a gel coat, then finish and prep to paint. When you work the door, be sure to allow for matching the door to the fenders - that may be why you're gel coat is lacking, the factory took it down to match the fenders before paint and removed the gel coat.
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Old 07-20-2009, 11:51 AM
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there is no gel coat on a vette body .
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Old 07-20-2009, 02:29 PM
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With all due respect to Shine -

Corvette bodies prior to 1983 were constructed of Glass Reinforced Plastic (GRP), made of fiberglass and thermosetting plastic (polyester resin). When epoxy is used it is GRE vs GRP.

The body panels are made from a moulding process. Starting with a buck or plug to form the desired shape, the panels are construced by:
1) Applying a release agent (wax or PVA) to the plug;
2) Applying a gel coat;
3) Formed using a hand lay-up process (applying layers of fiberglass mat and resin).

Boats are made with a finish coat of pigmented resin that is also referred to as a gel coat. This is not what I referred to when I addressed the crack and peel issue above.

When you remove the "paint only" from a corvette body, you will find that there is a smooth layer of hardened polyester resin "gel" coat on top of the panel. If you take a sander to it you will quickly work through the "gel" coat and find exposed glass fibers. Continue sanding to find more exposed glass fibers. Attemptimg to prime and paint over these fibers produces poor results, often the result over time is the paint cracking.

Your recommendation to use epoxy (or epoxy paint) produces the desired result because the epoxy is essentially a thermosetting plastic that bonds with the exposed fibers and once it is cured makes a durable paintable surface.

Common usage of the terms polyester, epoxy, gel coat, etc apply to a wide range of products and their uses. Thus there needs to be some tolerance for those of us that misuse the terminology in ordinary conversation.

Back to the problem at hand, when the paint was removed back in the '80s, was there any difference in the surface texture of the door affected compared to other areas on the car? Were there any dry "hairs" exposed?
If so, this would be what I was talking about. Either when the gel coat was applied to the plug it was done incorrectly or when the door was worked for final fit and finish, the gel coat was sanded away before painting.

Another possible cause, the GRP panels are pourous, and liquid materials can transfer from one side to the other. As an example, on my '71 coupe - the hood had been damaged slightly on the bottom side by the (you guessed it) air cleaner wing nut. Of course on top the dreaded spiderweb cracking appeared. Thinking that the cracking was caused when the hood was closed on the wing nut, I proceeded to sand out the paint and shoot with fresh color. Looked great for a while, then the spiderweb cracking returned. After fuming and fretting I learned that the GRP is pourous, the cracking was caused by paint lifting, oil from the engine compartment was passing through the hood panel and lifting the paint on top. Sealing the hood on the bottom was pointless, because by now the hood was an oil soaked sponge. I had to replace the hood to solve the problem.
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Old 07-20-2009, 03:23 PM
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what color was this gel coat ?
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Old 07-20-2009, 05:59 PM
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In spite of what has been said I would follow Shine's lead on your Vette. He has done more Vette's than all of us put together and knows what works and what doesn't...it's your dime.

Vince
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Old 07-20-2009, 07:16 PM
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i'm still waiting for the color of this gel coat.
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Old 07-21-2009, 07:39 AM
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Yeah--- What color was the gel coat???????????
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Old 07-21-2009, 08:36 AM
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What it all boils down to is the flexing of the body in the normal driving. Glass has to give and take and depending on the thickness of the paint in different areas of the body, it will crack.
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Old 07-21-2009, 11:48 AM
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Listen to Shine - He obviously knows more than me.
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Old 07-21-2009, 11:49 AM
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still waiting for the color of the gel coat .
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Old 07-21-2009, 06:26 PM
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Hey Shine....sill waiting?

Vince
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