That's what I use. Fill the tank with water. Leave room for two cups of acid. Top it up with water. Depending upon how much rust and the temperature, it should do the job overnight.Any tips on flushing a fuel tank with Phosphoric acid? I've got a litre of 81% here, I've read varying levels of dilution for very varied lengths of time but thought I might go with around 30%, sloshed around and lay at different angles for an hour at a time.