I do not know how it will work in the long run but I coated the areas that were going to be welded with weld through primer. I figured that it would protect the areas that are not actually welded.
For several other areas that can not be accessed I have used an Eastman product that is marketed for the inside of frame box sections. It comes with a long flexible tube and multi-direction spray nozzle. The product is pretty thin and seeps out of every crack and crevice. I had not thought about using it in the cowl area but it may not be a bad idea.
