First i sanded it it to get the old paint off, then 2 good coats of primer (just regular metal primer), then sprayed the whole thing white with regular metal spray paint from the hardware store. masked out the design and used the same brand of paint in green and black. Then as many layers of the toughest clear coat you can find, and have the patients to do! I am not too sure how it will hold out against the rocks though...
i painted a frame years ago just flat black, and some rims... but nothing like this. i paint at home on canvas and have painted a motorbike and other random things... but this was an experiment on technique. to get the lines really clean i bought some thin pinstriping for a car and pinstriped all the borders.. then clearcoated it over the pinstriping. the colors i chose while at the hardware store actually.. on the fly