I have 20mm and .50cal cans that have done 4 Grand trips, countless Salts, Salmons, etc, and they are all still looking good. Painted 'em all with Ace brand best-grade rattle-can enamel. Have white, blue, one orange for first-aid. Cleaning and prepping the outsides is the key. I washed grease and dirt off, then sanded 'em with 180grit on an orbital, then wiped 'em with lacquer thinner very quickly. They now have a few scratches on them, of course, but I have seen pricier powder-coated ones that don't look any better. Retouching one you painted yourself is a lot easier, too.
So many rivers, so little time..........