irk miller said:Oxalic acid is highly combustible and can even explode when mixed with certain oxidizing agents and silver compounds. It's also highly corrosive to skin. Phosphoric acid is a bit safer and does a better job converting severe rust.
I think you need to do a little research, any oxidant mixed in strong enough concentration is explosive and if used "as directed" is safe and problem free, in fact it is sold in many forms in shopping centres, one being a pipe cleaner and stain remover as well as a timber deck cleaner. As for Phosphoric acid being safer, they are both as toxic as each other and both are corrosive to skin. With Phosphoric acid being listed as a known carcinogen and having other health side effects.