Preparing rusty fender for paint

If the part is small enough or you hae a tank big enough, electrolysis is great. After cleanup you can prime and paint.

Blasting is great if you're set up for it. Leaves things shinier than electrolysis and usually just needs an etching primer to get going.

Grinding with a twisted wire brush winds up being my most common method, especially for spots. Drawbacks are that some existing paint is the very devil to get off (but you at least have a good roughed up surface for a new coat of a compatible paint to adhere to) and it doesn't always get all the rust completely out of deep pits. In the latter case I use Jasco Paint and Prep (others use Ospho) after the grinding, both of which are phosphoric acid solutions that convert rust and iron to iron phosphate, kind of like bluing a gun barrel. Great stuff, easy to use. Treat it like a coat of primer, scuff it up and apply whatever comes next.

One caution is not to leave any bare metal laying around without getting something on it for a primer, whether that's an actual primer or the first coat of your paint. It can get a flash coat of rust in just a few hours, and you'll have to go back over it before priming or painting. Once you've got the bare metal primed, you can decide whether a filling primer or a putty would be better for filling the pits.

Good luck!

The Conneticut department that writes specifications and contracts for bridge repair in that state has determined the flash rusting is not a problem.

There are four types of ferrous oxidation. They have similar molecular identifiers but each successive stage includes one more oxygen atom. Flash rust is the first. The second and third stage are the most destructive to the metal. They both produce microscopic worm holes that are nearly impossible to clean in the field and will continue to decay the steel after covered with paint. The only remediation is to use zinc rich paint formulas. The zinc will sacrifice itself and delay the steel from doing the same.

CT will allow painting over flash rust. I seem to recall somewhere that flash rusting can promote paint adhesion under most circumstances. Flash rusting over worm holes is not satisfactory but may have to tolerated because a genuine "white metal" surface preparation cannot be realistically achieved.