Zud, hey? My own supply of oxalic comes as a cleaner called Bar Keeper's Friend.

Assuming you still have some of the pure oxalic left over why not try the toothbrush slurry mix trick with it? Oxalic acid is not a highly corrosive acid such as hydrochoric and sulphuric so while I would not want to brush my teeth with it or get any in my eyes a toothbrush's worth used with some rubber dispoable gloves and safety glasses isn't going to hurt the paint on the frame at all.