This one was a tricky one to run an experiment on, because the best source of oxalic acid is Bar Keeper’s Friend (BKF), which comes in liquid form, and also in powder form. BKF is an acidic cleaner used to remove rust stains from bathroom fixtures, and so is likely ideal for removing rust from metal. To test its usefulness in removing rust, I masked off a section of the sole of a rusty block plane, and made a paste of BKF powder and a little water. I then applied it to the exposed portion of the plane sole. The paste dried over a period of six hours, and to be honest, I wasn’t really expecting there to be a good result here. No doubt using BKF with a scouring pad and some elbow grease would provide some reasonable results, but just having it sit there, somehow absorbing the rust?
Well, I have to say I was intrigued after scrapping off the dried crust of BKF. The surface rust had been removed from the sole of the plane, as is evident in the photograph.
I took this photograph without washing the de-rusted region of the sole with anything, hence the residual paste at the top and bottom. A close-up of the border between the rusted and de-rusted regions is shown below.
The oxalic acid paste really does work very well (it is stronger than vinegar), and I would recommend it for large objects that cannot be immersed in a liquid rust remover.