From everything I have read prep either makes it or breaks it . if it does get a rust flash on it , clean it up before doing it or it will turn green . I learned this on another piece I did .
I bought a XD9 sub compact today for really really cheap that had aquired surface rust on just the slide during a recent move from leaving it in the case in his unheated garage for a couple weeks while he got settled in , The guy had cleaned it off with steel wool and of course smoked the finish almost to bare steel in a few spots so It is getting it bead blasted today and getting the same phosphate treatment monday . Hope it turns out as good as the AR barrel which looks like new also
sure would be nice to aquire a like new XD9 SC for $250