Paint is a rust barrier, shielding steel from oxidation. Mark Podeyn of Action RV, Albuquerque, N.M., specializes in inspecting and servicing steel and aluminum horse trailers. We look for rust on all trailers. He described how rust can be cosmetic or structural. On a steel fender, consider a rusty scrape as cosmetic.
Structural rust is more serious, generated from moisture from the inside out.
Inside the trailer, the horse produces moisture as he breathes and sweats. Condensation damages steel that's not protected by paint. These treatments increase the steel's resistance to corrosion. If paint is scratched from a galvanized wall, you shouldn't see rust on the steel's surface. Aluminum alloy won't rust, but it will corrode.