its KREEM, and i wouldnt go do it yourself. if you want to try to get the rust out, try searching for "rust in gas tank" and "gas tank cleaning", a lot of people here have done it with various kinds of acids that dissolve the rust. kreem only works well if you can take off the tank and turn it around to distribute the coating evenly inside, its a SEALER not a cleaner. so unless you want to disassemble your 'ped to coat the tank, read up on older posts about using acid to clean it.
-Garret