I buy phosphoric acid by the gallon at a local chemical supply. It's a 75% solution, which I'm told is the highest percentage available in WA.
I mix 1/2 cup of the 75% solution with 4 cups of water to use to convert rust . I'm using it to aid in removing rust from pitted sheet metal and for storing cleaned metal to prevent rust.
One gallon from chemical supply is $15.
