Basically any acid will do the trick to a greater or lesser extent. On the bottom end of the scale would be white grape vinegar and all the way up to hydrochloric acid. The recommended one is always oxalic acide, but I didn't have any when I did this a few years back while renovating an old cabin boat, so I used some diluted brick acid (hydrochloric) that I had lying around and it worked a treat. apply with an old brush, leave for a few minutes and then wash it off. Handle with care and wear rubber gloves.
