[CC69Rat]

Filler over rust = bad idea. To save the metal you want to stop the rust and clean it / blast it / repair what you can.. then treat the panel to stop the rust from coming back. Epoxy prime the panel, then your filler if that's the route you want to go. Block it back flat and a good 2k primer to block and paint. Take a look at the Epoxy and primers from Southern Polyurethanes. They are easy to spray and very fairly priced. Their primers sand very nicely and the Epoxy is great.

The idea is to 'seal away' the bare metal. Regular primers and fillers are porous and will allow water to penetrate. Water and bare metal will cause rust underneath your new paint and ruin your work.