ProjectC4

Bought my '84 last year, car had been neglected and had a long term power steering pump leak which slung oil up under the hood. There were paint bubbles above the leak. Brought the car to a body shop that I have known of for 30 years for paint. While they were doing prep work I read in a thread here how fiberglass could get oil soaked and cause paint problems but if you grind down to glass & heat the part up with sun or a lamp the oil might come to the surface and could be quickly wiped off with a solvent over a period of time and that sometimes the only cure is to replace the part. So I warned the body shop owner about this. He didn't think it would be a problem since the C4 hoods were painted on the underside and oil should not get through the paint.

When I went to pick the car up the bubbles were already coming back on the hood. I kept the car for 3 months and did the power steering pump & hoses; water pump, radiator, heater control valve, all coolant hoses and timing chain. So now there are no more leaks (OK the oil pan but that's a job for spring). I brought the car back to the body shop for the hood & some other minor issues. They ground down to the plastic (Bondo) that they had put over the bubbles on top and also ground down to glass on the underside of the hood. They used a heat lamp but saw no evidence of oil. No darker spots, nothing. The new bubbles had lifted the paint & primer.

So I guess the volitile compounds in the oil are gassing off and going through the body plastic and making bubbles. They primed the bubble area and I'm going to wait and see if the primer bubbles (although maybe I should put paint over the primer to trap the gas since primers are porous). Put the car in the sun in the spring and see what happens, maybe the gassing will stop after a while.

Anybody have any other ideas, anybody ever fix this problem on an SMC body car? I read on here that SMC is polyester fiberglass, same as most surfboards were made of and that it needs a special resin to deal with the release agent residue left in the panels. How about grinding down the underside, patching with surfboard glass with the SMC resin in it then doing the same on top, essentially replacing all the glass? Or how about grinding down the top some then gluing a layer of aluminum foil to stop the gas then bondo over the foil? The body shop owner was worried about the different thermal expansion rates of metal and plastic causing problems with the foil.

Anybody have an '84 hood for sale within a few hours of NY?

oldalaskaman

I have heard your 84 hood wont fit anything else, but its my understanding that any other year hood will work on an 84, which will broaden your search options a bit, and help getting the best price. might be a good time to upgrade the headlights also. I've become a real fan of the later year lights. Seems with all that extra work, now would be a good time.

anciano

Polyester resin is used for most fiberglass boat hulls, too, so you can get the stuff at any boat place like West Marine. I patched some deep chips on mine with regular epoxy resin since I had some left over from various projects, and it seems to be holding up well.

I'm not quite sure what you mean by the "release agent" residue. I've never heard of this with fiberglass but that doesn't mean it isn't there. There is something called "blushing" which leaves a layer of chemical residue on cured fiberglass surfaces, and that has to be removed with something like acetone before putting on another layer of cloth/resin, but that doesn't sound like the same thing.

I guess if it were me and the spots continue to outgas and bubble the paint I'd cut out the spots altogether -- assuming they don't cover a vast area -- and patch with new fiberglass, not Bondo. It sounds like the paint shop is doing the corrective paint work for you so they are doing the difficult part.

Be sure to mix a lot of microbeads in the resin so that it is sandable after curing. These are available from West Marine as well.



Or, as suggested, find another hood.

ProjectC4

Don't know if they used sealer but I'll ask. They did contact their PPG paint rep who had heard of problems like this but did not have a solution.

ProjectC4

From what I have read on here (search SMC) the fiberglass was made in sheets by putting down resin then shooting chopped up fiberglass then more resin and that there was a lot of release agent used so it would not stick to the molds and that the release agent stayed with the panels so a special SMC type resin is needed for repairs. I also just read in the "Collectors Originality Guide Corvette C4" that after the sheets of SMC were rolled out uncured and left to mature for a couple of days they were then then pressed at 1000 psi between matched chrome dies then pressure was reduced to 800 psi while it thermoset for 90 seconds at 300 deg F. Then the press was lifted .05 of an inch, a urethane plastic injected and the press clamped down to 600 psi to spread the coating into any surface imperfectins. The panel was then cured at 300 psi. So I guess there's some urethane in there too.

The bubbles are in about a 10" oval. I was going to do any patching with glass but leave the finish work to the shop. I was surprised that they use Bondo not microbeads like with a surfboard. Although microbeads mixed with resin are a lot like Bondo

ProjectC4

The car was painted in July & August of this year. It took about 8 weeks, they kept it on a back burner and used it as fill in work. It was really hot this summer. I had it from August 'till Dec and there were no other bubbles that came up. These bubbles are in the same place as the old bubbles. Thanks Mike.

anciano

Yes, in the sense that Bondo is easy to sand. But I really don't like the stuff -- it has very little structural integrity and cannot be used to wet out cloth mat which WOULD add some strength. Bondo's website doesn't even say what kind of resin it uses (epoxy, polyester or vinylester) but just calls it "proprietary resin." Gee, that's helpful in determining compatability, and you have to wonder how well it bonds with SMC.

samsonb

It is my understanding the SMC resin is the same as fiberglass resin. But has something in it to help bite into the SMC. Most seem to use Evercoat SMC resin.

anciano

The FSM says it's a polyester resin, which is commonly used in boat hulls because it's cheaper than epoxy resin. OTOH, boat hulls do not undergo a thermal shaping process so maybe there's something different in the formula. If there is, it's probably in the hardening agent rather than the resin itself.

sailorsteve

If you're looking at replacing the hood, you have nothing to lose by experimenting with the one on the car. You could try sanding, prep, etc. as before but before you spray any paint or primer, paint a coat of epoxy (not polyester) resin over the problem area as a sealer. If you use the type of epoxy (e.g. WEST system) that leaves a blush behind, you have to wash it off before you paint over it. Sand thru the grits to get it smooth, then do your painting schedule. In my experience with epoxy, it seals pretty much everything.