As a teenager, I landed a job waxing a 47-foot yacht, and after washing it, there were still various stains that I couldn’t wax over. The man who ran the marina told me to run over to the paint store and buy some oxalic acid. I came back with half a pound of this white crystalline substance and, following his directions, mixed about half of it in water. Then I took a mop and wet down the topsides of the boat with the solution. No scrubbing; the stains went away! After rinsing the boat, I waxed it, and the yacht looked brand new!

Oxalic acid will get rid of many types of stains. Remember, though, you need to use common-sense precautions: Don't get it in your eyes, and make sure you rinse well.

– George Fogel