Suppose that you owned a really nice Eddystone M1917 and an Arisaka Type 38 carbine. Suppose you'd left them out of the safe because you'd been playing with them, and they were resting against a wall with their buttplates on a raised piece of particle board.
Suppose that this piece of particle board was on a basement floor that flooded and, while it kept the rifles actually up out of the water, the board itself got waterlogged, and when you looked at your rifles before putting them away you noticed that they had developed a bit of orange fuzz on the heel of the buttplate where it had been in contact with the damp.
Tools you will need: A cloth; some good protective oil and some cleaner/oil combo (I use Butch's Gun Oil and FP-10 respectively; everyone will have a favorite); a brush; a brass cartridge case (I've found a bottleneck rifle cartridge of about .30-'06 size works best for me); and a hammer.
Use the hammer to mash the mouth of the case flat. Lube the cancerous spot with the combo oil and use the flattened end of the brass case as a scraper to get the oxidation off; brass being softer than steel, it won't hurt the underlying metal. Then go to the brush and cloth and clean as normal and apply your protective oil. Et voila! Cancer removal.