Go to chemistrystore.com and look for Oxalic acid and also, try Sodium Percarbonate. There are recipes for concrete and vinyl siding cleaning. You can buy 50 lbs for around $1oo.oo as well as smaller amounts. These are the safer bleaching agents that work quite well. Oxalic Acid is as it is called an acid. This is the stuff you see on infomercials as "Oxy-Clean. Sodium percarbonate is an alkaline cleaner, probably best used for roof shingles.It will also nuetralize Oxalic Acid for areas in which they may apply a sealer or finish. Dont want it reacting with the sealer! I always start with the safest product I can use then if that fails, consider the stronger stuff. The thing is, these are safe for you to mix and effective to use. Plus, you are not paying for some one else's corvette if you do it yourself!