I wrote an article for The Fur Shed about this. Here's the condensed version.
First, pressure wash to clean the trap.
If there is any rust I treat the traps with OSPHO by SKYCO, available at Ace Hardware stores. This stuff turns iron oxide (rust) into iron phosphate which is an inert hard substance that turns metal black. it stops rust from further attacking your trap. You can paint right over it when dry. It doesn't hurt painted, galvanized or clean surfaces on your trap.
Get a tub your trap can stand up in and just pour the stuff over your trap. Excess goes in tub and you can scoop it up with a cup to treat places you missed. Save the left overs for next year. It takes abut a day to dry. This stuff is acid so use appropriate precautions when using.
You can coat the traps with speed dip the same way in a tub or rattle can it. if you have many traps to do spray paint has its drawbacks but for one or two maybe that is your route. Anymore than that I think dip poured over the trap is faster and gets better coverage.