I first heard of it a few years ago at a radiator repair shop. Its not that you don’t remove the rust first, you just don’t have to. Allot of online part stores carry it and a few sellers on eBay as well but for me, its the fact that I don’t have a garage to work in so if it cant be done on a Sunday afternoon, I cant do it (at least not the right way, I do consider myself a master of the Half-Ass Mechanic Methodology or H.A.M.M. for short).