Pin holes usually mean the metal is thinned, er, a bit too much. This doesn't leave much to weld onto, and often results in bigger holes, as thin metal melts away from the filler rod far quicker than thicker metal. Brazing, on the other hand, bonds to the metal surface before the base metal melting point. Braze the entire corroded area, then file to match the original contour. Do, de-rust line the tank internals afterwards.