...but i would would assume worst case scenario is that he would have to re-polish witch would take Approx. 5 min. The acid isn't going to eat into the metal but it might dull the outer varnish. The only reason i would suggest this is because it would %100 clean out all the contaminants and even the left over carbon build up that all of our plenums have over the years. I would think it would be a more of a guarantee then spraying it off IMO