As far as RUST goes It needs to be cut out and new metal welded in is best with that being said yes I have a good metal treatment for LIGHT rust. BUT The rust needs to be removed BY Blasting, Wire wheel or sanding. As far as Blasting media is concerned my favorite is Glass followed by Aluminum oxide or Black beauty.
Once all traces of rust are removed a Product made by Dupont can be used called 5717 and 5718 the instructions need to be followed to a T and thoroughly washed after wards with soap and water.
If the panel is rusting from behind It really needs to be removed and new metal welded in.
There are Products called Rust bullet or Por15 these products are meant to be applied directly over rust do not remove it as it needs to be applied directly over rust but you do need to take the loose stuff off and clean it good before applying it to work properly. I have not used these products & I have no experience with them.
I like the Dupont product, Its more of a Metal conditioner or metal treatment than a rust converter. A good rust converter would be a Product like Navel Jelly.
After treatment with 5717 it looks like this
After 5718 it looks like this after washing with soap and water and allowing to air dry
Most of this car was stripped with the pads posted above and then smaller ones followed by light blasting before metal treatment and application of Epoxy.
The adhesive patch I did some time ago
The gap & Cleico holes where cleaned of all traces of panel adhesive and Fiber glass applied over it Epoxy was applied by brush all over the inside.
The inside was also blasted prior to the Patch being made and installed.
If I where to do it over again I would weld it But i wanted to do the repair like this to show it could be done, To do it better next time I would have no gap and maybe fewer Clieco holes.
Is the repair text book? No, did it work? Yes, is it strong? Yes,
Is it gonna get any one killed? NO
Its a heck of a lot better than being filled with news paper and Screen and Bondo like I have pulled out of other cars over the years I leave that up to you to decide.