How to paint a badly rusted metal bulkhead door?

Its worth a try to paint it. Try the rustoleum spray paint or a type that is specifically meant for use on rust.

Maybe. You need to treat the rust first. Use a wire brush to remove anything that is loose. LocTite makes a product called Extend that reacts with rust to cure it into a paintable surface. Follow the directions on the bottle or can. Once you have the rust stopped you can fill any holes or dents with Bondo and sand them smooth. Use 2 coats of an oil based primer follow the directions on the can for time between coats. Apply an enamel paint that is rated for exterior use. Unless you are determined to use a sprayed product, I recommend applying both the primer and the paint with a foam roller. Because the surface of your bulkhead is unlikely to be smooth after rusting to the point you described, the texture left by a foam roller will keep the finish from being too shiny and showing all the imperfections.

Yes remove rust then paint---Need to treat doors with NAVAL JELLY then prime and paint like you want https://homequicks.com/how-to-use-naval-jelly-to-remove-rust here's product http://www.loctiteproducts.com/surface-treatments.shtml you can get it online or any hardware stores