I actually don't know what kind of acid it is, the package doesn't say. I bought some stainless steel blackener from caswellplating.com from their antiquing section. It's easy to use. I sand-blasted the heads, blew them off with air, dipped them in some alcohol to clean them, dipped them in pure water to wash that off, then a 3-4 minute bath in the acid. Let them drip back into the acid bucket, put them on a towel to dry for 20 min, then repeated the process. After the 3rd dip and drying, I sprayed them with WD-40. And after that dried I did the paint fill.