If you have some heavy duty cleaning, or need it done fast, muriatic acid. It can be found with masonry supplies. THIS STUFF IS DANGEROUS, and if you don't use it outdoors, with gloves,respirator, and googles YOU WILL DIE!. Well maybe not die, but you can only use it outside, and you need to know what you are doing. I am not sure how much to dilute it, I have seen it used at 100%, and 10%, and both work great.