I have had at least a dozen painted rifles useing most types of coatings, all done by people who do it for a living, heres what i have found
Metachol, done only by ARS, VERY durable, can be done on small parts and in very hight temp aplications, somewhat limited in color but an extremely nice product.
Norrels Moly, nice coating as well, not as durable as some but still a nice product, havent seen a wide variaty of colors but lots of people do it so time frame is good.
Duracoat, it seems to be the all round winner for me, lots of colors, can coat abourt everything and after a few weeks very tough.
Ion Bond, only limited to the AR BCGs i have used, probably the best there is on steel , its Rockwell 90+, the drawback is availability, use on only certain types of materials and cost,
As always your millage may vary