what i do with my metal ramp flaps is first sand it with 1200 grit wet & dry, then prim it with etch primer. i let it cure for 24 hours then i paint it with high temp gloss paint & bake it in the oven for about 30 to 45 min at the max temp that the paint is able to withstand.
the result is very flexable, very shiny & a very strong finish. (you know how much the ramp flaps flex when you fit the ramp to the playfield.)