This afternoon, at the lab, I removed a ferric chloride stain from the workbench with a paste of citric acid and water (given 2 hours to work).
Reading up on the chemistry, I see indications that a mixture of citric and phosphoric acids should work better...
...which means Coca-Cola should remove ferric chloride stains. Has anybody tried it?
Also, oxalic acid is supposed to be very effective. It is the active ingredient in Bar Keepers Friend powdered cleanser. (Bar Keepers Friend liquid is citric acid.)
The idea is to convert FeCl into another iron salt that is light-colored and highly soluble in water.
Any other favorite remedies? (Other than to etch the printed circuit boards with ammonium persulfate instead?)