Hard to reach rust quarter panel

Interesting, completely edited the original title and all of the posts. Well you can read the quotes so my previous response makes sense.

Media blasting is an option, so would be rust encapsulation like POR 15 or others from Eastwood. Rust conversion with a phosphoric acid based rust converting product would also be an option, to be followed with encapsulation.

If there are areas that are more than just the surface rust shown you’ll have to replace that section of panel, so opening it up from the outside which will allow better access before the patch is welded in.