You need to use an etch primer, which can be found at automotive aftermarket stores. If you just put paint over bare metal, it will scratch off, very easily. The
etch primer, "bites" into the bare metal, than gives your primer, and/or paint something to stick to. Epoxy paints are the best solution, but money, equipment, and
proper preparation are needed. Etch primers in the spray can work pretty good. They are recommended for touching up small area's on vehicle's
but they can be used for prop builds as well, and work fine. you just need to make sure that all rust is removed, and cleaned with a metal prep, or
in the very least lacquer thinner ( but I didn't say that! ). Rust! The cancer for cars, or bare metal. you can clean and prep and metal surface to
perfection, but once it starts, it can have a slight chance to return. I've seen it do weird things, not trying to worry you, but it just does!
Depending on the durability your looking for, and if it's going to be inside, as apposed to outside. You could prep the bare metal with Lacquer thinner ( cheap easy way! )
than use a etch primer, than regular primer, sand smooth, than paint.
It can get difficult to explain proper procedures, in a forum. If anything check your local automotive paint store. Now! if you tell them what you want to do, and they have no
clue what your talking about, just turn around and leave! Paint reps are a dying breed. I know because I am one.