I have beeen reseraching this cause if Phosphoric acid is the active ingredient, some of these companies are blatantly ripping us off. 1 qt for $55 is ridiculous (por-15 for example). Not sure where to buy straight up Phosforic acid, but Zep and Behr have some rust removing products whose MSDS sheets show them to be almost 50% phosphoric acid, and a gallon of the stuff is less than $10...