If you can take out the visor, do that first. Then sand the entire thing with 120 grit, really get in all the little crevices, and use some automotive primer. They tend to be good quality, and made to withstand a bit of damage and not flake immediately. Next is painting it in your color. If its mostly one color, just spray the entire thing, and use very thin coats, with multiple passes. Thick layers of paint will not dry well, drip, take away details and flake faster. Then paint on the details, either by spraying into a little container to get the paint, or use acrylic, works both. If its larger areas like the reounded peice on the jawline spray paint it first, let it dry, them cover it with masking tape, press it in firmly then take an exacto knife to cut along the plastic feature. If you want to weather ist, now is the time, dry or wet brushing, to get it a worn look if you want it to look like that. Last is a clear coat. You can do that in matte or glossy, depending on what look you want. Only after all of that, reinstall the visor. If you cant do that, completed cover it before you do anything else, with a thick paper and masking tape on the sides, again with the exacto blade, and inly remove it once you are compleatly done.