powder coat them?! holy crap i've never heard of anyone powder coating their wiper arms hahahaha

just pull em off, grab some decent grit sand paper, prolly like a 150 grit, sand em pretty good, and use regular primer, or like a rustoleom primer(primer more for metals) and then use any kind of flat black paint you want, and shoot some clear over it. the clear will protect the paint and will take the abuse of hard winters. no need for any high temp paint or anything... how hot do your wiper arms get? :eh: