Welcome to the forum. It would be great if you would set up a signature with at least your first name, type of bus and location. That way, we might be able to help you better.The first thing we need to know is the type of bus. Second thing, pictures would be great. I assume you have access to the inside where the corrosion is taking place. You need to stop both the steel and aluminum corrosion (steel takes something like "metal prep" and aluminum takes a different solution - we used to use a product called "Alumaprep" or something like that). Then, you need to use some sort of special aluminum primer on both sides of the affected area. If you could find zinc chromate, that is the best, but I think the EPA made them take it off the market. Someone recently noted that it might be available from a marine supplier.Depending on the amount of damage, you could try to clean out the holes, and use fiberglass to fill the holes. I would not use Bondo, You could try the Bondo with fiberglass fibers. Before you apply the filler, you should use something like a ball peen hammer to indent the hole so that you have a good surface to grip to. Again, this approach depends on making sure that the corrosion has been arrested and the surface properly prepped. I think it is best to put the fiberglass material over the proper primer. We always prepared steel surfaces with PPG DP40 before applying Bondo.SaltX is great stuff for protecting against salt damage. Lots of Bonneville folks use it. I am not sure that it is the correct product for this application.To do it right, I would visit a GOOD automotive paint supplier and see what they recommend for the treatment, primer, and filler products (the industry has changed a ton since I was involved).Jim