VinceP

did all C1/C2 have a gel-coat
Thanks Vnce

Islander21

Don't know about C1s, but C2s did not get gel-coated at the factory, no reason to. Gel-coating is an accepted way of sealing in fiberglass repairs, however, especially if the body has lots of wounds like my 66 does.

csherman

early c1's didnt
they got a brown primer then paint

BarryK

no gelcoat from the factory

mike coletta

The panels were "press molded", not gelcoated. As the panels were pressed, the resins were drawn to the top and bottom surfaces, creating a smooth finish on both sides. Gel coat is what is used during the normal fiberglass molding process, ie: boat hulls. Mold release is applied to the mold, then gel coat (polyester resin), and then coated with several layers of fiberglass mat/cloth, with resin. The result is what you see on a hand layed fiberglass replacement part. The inside, or unfinished side is very rough, showing the fiberglass mat (like the inside of a boat). Gelcoat on a Corvette is a way to "surface" the fiberglass. It has NO strength, and is purely cosmetic. With the products available today, gelcoat is old technology. Much more solid results can be had with more modern products. I switched over to Epoxy surfacers about 6 years ago, and find that the holdout is much better. Here are some facts:

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.(I think that Corvette Image uses vinyl esters in the manufacture of their parts).
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.

I could go on, and take no credit for the above information. It was part of a study done by a major resin manufacturer. While ALL repairs cannot be done with epoxy, my thought is that the ones that can.....should. I use a high quality body filler (low shrink). I use only ISOPHTHALIC poly resins (mold maker resin, low shrink). I use epoxy primer/sealer/surfacers under the paint.

If you have a car with little or no bodywork, then I suppose that one of the polyester surfacers will be ok. I used them for years. If you're doing a dark colored car, and have significant bodywork, I would use the epoxy vs. gelcoat or polyester surfacers. BTW, the volumetric shrink on polyester (not isophthalic), is about 5 percent over the first 5 years, and 2 percent over the next 2 years, which is why you see sanding scratches, bodywork, etc., coming through a paint job several years after it was painted.

These are my opinions and observations. I've painted hundreds of cars. Problems don't show up on every car, and as I've said, I used polyester products for years.....some with problems, and some without. I posted this for information only, and is certainly not intended to be for everybody.

Mike Coletta

rick66

Good question by the OP. I have wondered about this myself.

Awesome response by Mr. Coletta. I will print this one off and file in my "future use" file.

ChattanoogaJSB

Thank you to Mike- I've used Epoxy in any recent paintwork as a base layer because of the adhesive properties it yields to surfaces above and below it, but I've never known as much about the strength and shrinkage aspects as you've shown.

I understand your use of high-quality fillers, but do you use a sort of "high build" epoxy for blocking one out? The epoxy I've used (the old ppg DP90 comes to mind) is paper thin when cured and shows everything, and is "non sanding."

wombvette

True answer to the question. In general NO gel-coat, however, some parts of very early production, like 53, 63 etc. or very rare options that had very limited runs, were done with gel-coat. And there were some 54 bodies that were done with gel-coat as an experiment.

mike coletta

I use PPG Global D822/823. It's a 3:1 mix. It is VERY high build as a surfacer. Then 3:1:1/2 for sealer, and 3:1:1 for primer. It sands like butter. I usually apply two wet coats, over 80 grit scratch, block with 80, apply two more coats, and block with 220. Shoot as a sealer, DA with 320, and paint!! One of the nice features is that you can spot it in if you have a particular spot that you're working on. It dries to sanding in about 1 hour. Pot life is short, about 2 hours.

Mike Coletta

ktchir

Great information!

knockbill

thanks for the great info,,, i was all set to use slicksand, until i read your post...

i read ppg's instuctions, and have a couple questions,,, how long can the car stay in this primer before the top coat is necessary, or doesn't it matter? what do you use to clean the paint gun? will laquer thinner work, or does ppg have a special cleaner.,,,

thanks again for your help....

mike coletta

8 hours to topcoat, or sand. I use a 3m fine or ultrafine sanding pad, or you could sand with 320/400 dry or wet. I suppose that you could leave it for years, and then sand before topcoat. Here is the P-sheet from PPG:

http://www.bapspaint.com/docs/psheet...obal/EU091.pdf

Lacquer thinner for cleanup.....but before it dries. NOTHING will cut it after it flashes....except sandpaper.

Mike Coletta

knockbill

thanks mike.... i had found the ppg p-sheet,,,
excellent info,,, i appreciate your willingness to share your experience, and your time with us....

DansYellow66

It seems that I remember hearing once that (at least on C2s) they initially shot a finer fiberglass fiber on to the outer mold when starting a new panel to give a smoother finish. And then switched to the normal size fiber to build up the full panel thickness before closing the mold for initial curing. Does anyone remember anything like that?

knockbill

i'm sure they kept trying to improve, and strengthen the glas over teh years, so that makes sense,,, i remember repairing panels on my 68 years ago,, and the fibreglas on that car was smoother and more refined than the glas on the 56 i'm working on now...

JohnZ

Nope. Photo below shows the original high-tech process for making a midyear upper surround. Lay mat preform in mold, pour precisely-measured () amount of resin on the preform, close heated mold, remove surround 5 minutes later, trim, and ship.

ifitgoesfast

Wait, now I'm confused. The file name of the above photo is 47697643d1338320853-gel-coat-67surround75.jpg

"gel-coat"

Is it, or not?

JohnZ

Nope. That's what someone else thought it was.

There was no gel-coat on any Corvette panels after 1954 (except for low-volume hand-laid parts like the wheelhouse inner panel caps on big-tank midyears).

ifitgoesfast

I'll have to post a photo later when I get home. My paint, after long storage and a bad previous paint job, had cracked, bubbled and peeled on several areas down to some kind of reddish brown layer of something. I don't think it's primer but really don't know. It's the layer on top of the fiberglass, and it's most likely around the whole car (since on several sections). Any clue? Is it primer?

JohnZ

Yes, it is. The factory used red oxide primer, with gray primer sprayed wet-on-wet on top of it, and most of the gray was sanded off before it was topcoated. What you're seeing is the red oxide primer.