hydrochloric acid will cause rust and attack the healthy sheet metal, because it is an oxidixing acid, a reducing acid like sulfuric acid of phosphporic acid will reduce the iron oxide to free iron and water (the oxygen bound in the iron oxide reacts with the hydrogen ions (protons) given off by the acid.) and not harm the healthy sheet metal
a 10% solution of sulfuric acid should yield the result you are looking for, you should treat with marshals solution afterwards to prevent further rust.
if you are not chemically inclined just stick with rust-mort or simmilar products..
a 10% solution of sulfuric acid should yield the result you are looking for, you should treat with marshals solution afterwards to prevent further rust.
if you are not chemically inclined just stick with rust-mort or simmilar products..