No matter what you use it's probably not gonna hold up great. Like other hardware, those arms are usually either plated, anodized, or powder coated. I guess if you really wanna do it you have a couple choices. Take it to someone who can powder coat it, might be hard to find someone who will do such a small single piece though. The other idea, probably what I would do if I wanted to try it, is just to make sure you sand it down really well, don't break the paint that's on it though, go with 6 or 800 grit, get rid of all shine, and just go with enamel spray paint. Others may have other suggestions, but nothin's gonna be great unless you get it powder coated.