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Mine is a Calif car too, other than some surface rust she is clean, except under the radiator like yours. I wonder why?
In my case, the rot was directly beneath the drain plug. This suggest to me that, over the years of (hopeful) maintenance, repeated contact with drained fluid affected that area more than the surrounding parts.
When I discovered the problem, I hammered and tapped all around the support, breaking away the rotted 3" cross piece from the lower portion. After determining that everything else was still solid, I cleaned it all off with solvent, then made a fillet from MarineTex, bolstered with several layers of biaxial fiberglass cloth and an additional layer of epoxy extending beyond the fillet. For 'CYA,' I added some fillets in the vertical portions which looked as though they may eventually begin to fail.
Although I believe the repair was effective, I consider this a temporary fix. At some point in the future, I'll replace the entire support.
Mine was the same as Snoopyvet and funnily enough I was thinking the same thing when I washed the car. I believe that when the water drips down inside the body from around the headlights and front of the hood it pools by the bottom of the radiator support especially near the its corners. This coupled with no internal rust protection from the factory means they become rust prone.
Perhaps a drain hole strategically placed under the support would help? I blew fish oil through the inside of the support on my car to help keep the rust away.
Mine is still that fugly green color. No rust at all, luckily for me, because I didn't know a thing about Corvettes when I bought it twenty years ago, out of dry storage.
I believe that when the water drips down inside the body from around the headlights and front of the hood it pools by the bottom of the radiator support
That makes a lot of sense. The unit seems especially vulnerable to moisture from that area.
I like the fish oil remedy. Probably helps with the engine's cholesterol level too, eh?
Mine is a deep south car and has a pretty clean windshield frame, birdcage and frame. Having grown up outside Detroit and living many years in the Midwest (Detroit/ Cleveland/Chicago) before I moved to SC; I am very familiar with the rust issue on ALL cars that spent much time up there. Assuming they were driven year round. Right after we moved here in 2000 I took my Cherokee, that I had brought down here from Ohio, into a shop for some exhaust work. Guy puts it up on a hoist, see the rusty undercarriage, and says, "What the heck is that!? Has this car been sitting in a lake?" I say, Nope, its just from NE Ohio.