My husband's car had developed a large rust hole in the fender under the gas cap.
It needed to be fixed before the winter, and my husband hadn't had the free time to repair it himself; and I didn't want to pay the $150-200 to have it done by a body shop.
So I figured I'd do it myself and surprise him.
Step 1: scrape and sand
The first step was to scrape off the loose paint surrounding the rust spot, and to sand off most of the rust.
I found, under the outer layer of metal, what I thought were additional layers of rusted, chipping metal, was actually dense foam padding. I scraped the rest of the metal chips off of the foam. I then taped off the area, and sprayed it with a rust treatment.
Following the directions on the can, I allowed the rust treatment to remain on overnight before I continued. (Rust treatment goes on clear, dries black.)
On day 2, I again sanded around the area, removing paint around the hole to prepare it to be built up. For rust and paint removal, I used a 60 grit sandpaper.