If you plan on using them rims like that for a while or doing any off roading with them I suggest the following:
1. wash the rims with a mild soap and a green pad.
2. sand them with around 320 grit paper and knock down any issues
3. spray a primer base (you can use spray paint or a gun)
4. wet sand with 800 grit to smooth out the primer and take off any shine
5. spray your gunmetal in light coats make several passes
6. put a good amount of clear over the top, they make gloss, semi, and flat clear in sprays now.
If you want a high shine use 1000grit and wet sand between clear coats and do a final polish. I have used this method many times on bikes. cars, and off roaders and have had no issues of scratching or such. Just some points that I have used. Hope it helps.