A warning about Oxalic Acid a chemical which I've been using as a cleanser for over 30 years in my line of work:
Here in the UK it comes under the 'Poisons act' and is not at all easy to buy 'off the shelf';
it's a white salt-like crystal which is dissolved in warm water or Methylated Spirits.
It does not burn or sting but you should still wear latex gloves when using, also rinse the job off with clean water afterwards.. most importantly wash your hands with soap and water after use
otherwise you could risk a severe bout of vomiting and illness if it gets near your mouth.