I use a product called "Picklex" that is acid-based and available through Eastwood or the Autobodystore.com site. It can be used as a rust-converter to some extent but is best used as a protective coating for bare metal and as a base for primer and paint. You simply spray it on from a pump-spray bottle and use a paint brush to spread it around for a few seconds. I have a pickup bed that I made over 10 years ago in my garage that was sprayed after fabrication but still looks like it was fiished yesterday. When you're ready to paint, simply scuff the surface lightly with a scotch brite pad and prime/paint afterwards.

For priming situations as you mentioned, I still use my 2-part urethane primer but apply with a brush or roller depending on the part shape, location, etc. You're going to be sanding the primer later anyway so a few brush strokes are not an issue.

Have fun.

- EM