i absolutely love por 15 - 2 coats is best but you cannot scratch it off if you wanted to. gotta prep it right tho: no loose rust (wire wheeled works good) sandblasting is best tho.
this 29 aa frame took roughly 1 quart (about $40) and some cheapo $0.78 wood handle brushes that i just threw away when i was done - you cant clean them nor get it off your skin - wear THICK rubber gloves - the paint goes right thru those blue "doctors" gloves - dont ask how i know that one lol. dont open the can either, use 2 self tapping screws in the lid and use one to pour paint out into a paper bowl (eats foam) and the other as a vent to pur faster then immediately seal up the can with the screws and start painting a little at a time. sets up rather quick but takes 24hrs to fully cure. it kinda self levels so you will not see brush strokes which makes applying it a breeze.
i suppose the only thing about it is it comes out very glossy but dries smooth as glass (granted your metal is smooth) BUT you can paint most paints over it if you rough up the gloss some so por15 it - protect it forever - then paint with desired color. POR15 doesn't like sunlight/uv at all hence a top coat is best but the frame shouldnt see much sunlight.