ALUMINUM WILL CORRODE, and it corrodes into what looks like a white powder!
Camino LS6 wrote:Excellent as usual, Tim.
I'll have to pay special attention to all of the front clip related info in this project. At some point, I will have to remove the front clip on my car to get at the birdcage and rad support - as well as the reinforcements under the clip. The reinforcement panel at the front edge of the hood opening on my car must have some rot going on at whatever hardware attaches it to the clip. There are small, round bumps at regular intervals showing on the outer surface of the hood surround at that location. How is that piece attached?
I believe it's bonded in place using adhesive. It's peculiar that you're seeing what sounds like blistering of the paint. While this fairly common on steel bodied cards, I'd be surprised to see it on a Corvette - the fiberglass panels should be really resistant to blistering just because they're so thick.
Do you have any pictures?
-- edit --
Actually, now that I think about it a bit more, there may be a pair of rivets on either end of the reinforcement plate that are countersunk into the fiberglass. This is sticking in my memory for some reason, but I'm not positive they were there. I checked my photos but no dice and I've since scrapped my original front clip. Perhaps someone else here can verify.
But, even if the rivets were there, they should be aluminum rather than steel so it shouldn't be a rust issue.
The bumps you are seeing are, in fact, aluminum rivets that have started to corrode from moisture under the front structure getting to the rivets. This is very common on C3 Corvettes. Once the moisture reaches the rivet, they start to corrode to something that looks like a white powder, and the rivet begins to actually expand like steel does when it rusts. That is where the "lumps" are coming from. For those of you in the North, it is similar to what happens to your blacktop roads when small pockets of water freeze under the road surface. They creates frost heaves and actually raise the blacktop above the frozen water. After the water under the road surface thaws, the cavity shrinks, leaves a void and creates a Pothole when you drive over it. Fortunately, the rivet never "un-corrodes", but it also means the lump never receedes.
The repair is not pleasant. One form of repair (the best one) is grind out the rivets and replace them. Since these rivets hold the fiberglass front clip to the metal framework, they need to be replaced one at a time. Simply sanding the lumps off or glassing over them will NOT correct the problem, it will return, and it will be after
you have painted the car. Another option is to grind down to the top of the rivet head, lightly scuff-polish the head of the rivet with scotch bright to remove all the corrosion, paint them with an aluminum primer and then re-glass over them. They are fairly thick headed rivets, so if they are not too far gone, this may work, but do not polish them too far. I understand that later model C3's had the front clip bonded to the metal framework with the newer epoxies.
If you are wondering how I know all of this, I have the same problem with my front clip. I have discussed this in length with my body man, John Jamison of Jamison's Custom Corvette in Charleston, SC. He has see this hundreds of times and has made these repairs. Even after the repairs are made, the rivets are still going to pick up moisture from underneath, so they can again begin to corrode and the problem will reproduce itself. As part of my restoration, I will be sealing the crap out of the underside of my front clip to prevent moisture from entering this area and allowing it to corrode again. It took mine 30 years to get where it is now, so it will take another 20 or so years to reproduce the problem, more if I am successful in sealing the moisture out.
Hope this at least helps, if not comforts.