I have recently been employed by a shop that focuses mostly on vettes. My team and I are all familiar with fiberglass and doing repairs on those pieces. We are not however familiar with gel coat. The owner of the shop is pretty well familiar with building vettes from the bare bird cage. He gave us some guidance through the gel coating stage since he has done it before.

The build started on a 68 vette birdcage. The body was lined up, screwed together(to hold in place) and bonded together with 3M panel bond. The body was stripped before we got it and we aren't sure if any chemicals or blasting was used in the process. I did however find evidence of gouging with a DA sander around the vents near the back window. The whole body was hit with 80 grit before we began to mud the car and begin contouring the panels together (contouring all body lines and corners to meet). All body lines were sharpened and panels straightened. We had rage gold on hand but ended up using mostly 3M platinum to build and 3M platinum plus to do a finishing wipe. We removed screws holding body together and filled the holes with a product called "vette panel adhesive", sorry I can't recall who makes it at the moment. A few spots were filled in with "icing" for a filler pinhole here or there or very small dings and scratches. Fast forward a bit and we are ready to gel coat. I want to say we gel coated over 320 but again, cannot remember for sure. I'll come back and update with specs on that. I was not in the both at the time of gel coating as I was diverted to another project of priority. Two of our guys ended up spraying 3 coats of 'Fibreglast' brand gel coat setup(first mistake IMO). The last coat was reduced slightly. We let it cure outside for about a week and a half before we began blocking. We blocked with 180 to prep for one more coat of gel as we took a lot back off in some rougher areas. Then applied the last coat after hitting with 320 the 400 wet.

Fast forward again to it being blocked down again the same way stopping at 320 and we have a few cut throughs in certain spots or on edges. A few pinholes were plugged with Komdi putty(brown that gets applied heads up, no mixing).
We put 2 solid coats of K36 on the car. Blocked once again, spotted cut throughs in with a thin coat, followed by a thin coat over the whole car. This again was blocked with 600 wet.

We use all PPG products for spraying. It was sprayed torch red(shop line) and cleared with 2021 clear setup. We let that cure out for about 4-5 days before jumping into it with 1500. A few spots needed 1k. We came back with 15 on the whole car followed by 2k. This was buffed out with a 3M white foam pad and 'G3' compound followed by 3M polish with blue 3M polishing pad. The car looked like glass over.

The car sat for a few days as we were blasting and blacking out headlight bezels ets to begin reassembly process in mechanic shop. We came in one day and noticed that it had sucked in exactly where the metal(not sure exactly what it's called) brace had been riveted on the underside of the nose section above the headlights. It was as if it got to hot from weather heating up along with cutting/buffing and sucked in revealing dented looking spots, but again exactly on the rivets. We began inspecting other areas and noticed some very tiny bubbles in places(not the kind from poor adhesion of paint) but micro air bubble looking spots. We talked to a paint rep who decided it was from the gel coat. The owner of shop claims that particular gel coat doesn't cure by gassing off after talking with reps from Fibreglast. We decided, with advice of rep to put heat lamps on every panel that was gel coated to bring everything that wants to gas to the surface. We managed to cut and buff those tiny bubbles out after the heating process. We did unfortunately, find that everywhere a screw had a been, now felt like a mild knot on the panels. We couldn't get them to go away, and today finally came to the conclusions of busting them open.

To sum this up, my questions are:

1. Why did each of these things happen
A - rivets sucking in
B - tiny bubbles surfacing
C - knots where screws were previously

2. What can we do to prevent again?

3. Gel coating tips, or advice?

Sorry for the long post. It's my first on here and really have never had this many issues before as my experience has been mainly on metal cars or smaller glass pieces such as hoods, bumpers etc.

Okay, to try and answer your questions I need to ask a question.... Why are you guys gel coating at all and more importantly who ever said you could gel coat over any type of body filler????? Another question, was your gel coat one rated for an "air dry" cure application, since most are not?
Just double checked this with a couple of sources.... that car indeed never had a gel coat.
Check out this discussion...
http://www.corvetteforum.com/forums/pai ... -coat.html

A rep said that it does work over filler. We were gel coating for that reason in the link you posted. To basically seal everything up. It was not rated for "air cure".

Gel coats can't just can't go over "any" filler. The filler has to be stable under the shrinking gel coat. That was, I'm sure compounded by not having the right gel coat to begin with. You either have to have a gel coat that self waxes (the wax is compounded in and floats to the top as it dries) or you've got to put a wax over it after application. I understand the need to establish body surfaces again I just don't think gel coats are the answer especially on press molded fiberglass that never had it in the first place. (And, yes, I know this is a hot dispute in the Corvette communities)

Here was some facts released by one of the major resin (and yes, they make ALL types of resins) manufacturer a few years ago....
1. Gelcoat is polyester based, as is Slicksand, Featherfill, and a host of other high build surfacers.
2. Polyester and vinyl ester products shrink up to 7% volumetrically and
because it continues to cure over long periods of time this
effect may not be immediately obvious. This is what causes the "print through" or "dieback" effect on repaired areas. Epoxy shrinks 2% TOTAL (approx 90% during the first 5 days).
3.As a general rule epoxys are twice as expensive as vinyl esters
and vinyl esters are twice as expensive as polyesters.
4.Epoxys have performance advantages over polyester and vinyl
esters in five major areas:
- Better adhesive properties (the ability to bond to the
reinforcement or core)
-Superior mechanical properties (particularly strength and
stiffness)
- Improved resistance to fatigue and micro cracking
- Reduced degradation from water ingress (diminution of
properties due to water penetration)
-Increased resistance to osmosis (surface degradation due to
water permeability)
5.Epoxys have far better adhesive properties than polyester and
vinyl esters. The superior adhesion of epoxy is due to two main reasons. The first is at the molecular level, where the presence of polar hydroxyl and ether groups improves adhesion.
The second is at the physical level - as epoxies cure with low shrinkage, the various surface contacts set up between the liquids and the reinforcement are not disturbed during cure. The result is a more homogenous bond between fibers and resin and a better transfer of load between the different components of the matrix.
6.After a cure period of seven days the tensile strength of the epoxy is 20 to 30% higher than those of polyester and vinyl ester.

If you notice #2. is probably why you have all of the problems related to shrinkage in your list. Prevention of problems in the future probably just comes down to spraying a properly curing gel coat and using more compatible filler underneath. I wouldn't even use a regular filler, I'd repair with a regular polyester resin and finely chop up mat to put in it, in simple terms, a fiberglass fill on fiberglass. It would all shrink together.

That's what I'm beginning to think. We talked to a rep with PPG who advised us to use a polyester spray filler in the future. We may try that and skip gel coating all together. That is making much more sense now! Thank you very much for the advice you have put in!

Yeah, I really have mixed feelings about the use of gel coats. I was a big proponent of them for years but as I have worked on more and more kit cars, body kits, etc., I've changed my mind about their long term performance. I just think there are better ways to get the surfaces you want without the gel coat process.

What have you found success in? We can't afford to keep re painting. Would just a basic primer sealer get the job done?

Well, I've had luck in the past doing again a "like on like" repair of what I mentioned before which was to repair fiberglass with fiberglass, then sealed up the surface with epoxy, and finally did any reshaping of contours with Slick Sand. In my next project like that I think I'm going to go the same way but try out the Tamco high build which is mentioned in this thread... viewtopic.php?f=5&t=24244&start=0
That would replace the Slick Sand for getting contour surfaces back. Chris here at the site had me try some of Tamco's clear and I must say I am impressed.