I am working on a 01 Bonneville that had an open rust spot about 1-1/2" in diameter just below the Gas door. It was also dented 1/2" deep. I prepped the area with a prewash, ground the rust to bare metal, except as I did so another small rust area began to appear and grew in size as I continued to grind. Must be rusting from the back side of the fender and is inaccesable due to an inner fender. I used Rust-Mort and treated the rust as per the directions. I was also very careful not to get any on the paint. After a full 24 hours, etching primer was applied. Next Rage Gold to fill the dent followed by S/W NP75C Ultra Fill Primer.
I didn't find the right solution from the Internet.