Thanks Tommyzoom, I'm just collecting info on the web about all these acids, and their different names and strengths. It's a minefield. We only sell 'high strength' acid, but they don't tell you the strength!! I don't want to keep buying stuff that does not work. Gisela's web link sells acid for £10 with £20 p&p. The cleaning guy said he could sell his 'high strength' acid to me for £60. I don't mind paying £100 if the stuff works. You are quite right Tommyzoon, having the right materials is the key to this dilemma. Cheers for your advice. Hopefully I will be posting a success story in a few days time.