I am restoring a truck with moderate rust damage in some areas. Specifically in the rear wheel arches.
There were holes several inches in size which I patched using a sandblaster and copious quantities of aluminum flashing, JB weld, and rivets. After coating with speedliner it feels pretty solid.
I need a plan for the top side of the wheel arches and inside the fenders. Surface conditions in these areas range from rust scale to factory applied primer (and the inside ends of about a zillion rivets). I cannot access these areas with any sort of wire brush or abrasive blast so my surface prep options are limited. I can spray thin liquids onto these areas using a wand type sprayer. What is my best option for treating these surfaces to halt future corrosion?
A second related question. Once the inside surfaces are treated I am considering filling the void areas inside the fenders with a 2 part closed cell expanding urathane foam to block out moisture, absorb sound and vibration, and give the fenders a more solid, less sheet-metal, type of feel. I am concerned however that over time the foam may shrink away from the metal and create voids which may trap moisture. Can this approach be used successfully? If so, what are the products and techniques needed to make it successful?