Ben,
Like that super soft. Course 8" means trimming the disks for the five inch Makita.
BUT the soft rubber disc could be used in the hand, by the hand, that is.
Just got handed a bottle of "nickel-safe water-cooler cleaner" that lists phophoric acid.
Tried some with a scothbrite pad on the piece of the mast bottom I had to cut off.
Got black stuff off with no reaction from the aluminum, except it got bright and clean in a water rinse. Way down there on the bottom of the mast there seemed to be some recognizable translucent anodize left. Totally gone everywhere else.
Acid doesn't seem to have bothered it or surrounding.
Does look like a lot of scrubbing ahead! The mantra, don't leave the acid on the aluminum, rinse quick.
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Assume dark corrosion is Fe 'leaching' - also reacting to salts.
"Phophoric acid has a unique property of dissolving iron oxide quickly while etching iron very slowly." (Bob Neidorff)
But of course, if that is what happened in that little experiment, then the dark stuff is actually iron oxide and is a surface phenoma (we'll see if there is any pitting ) so far as the apparently un etched aluminum mast is concerned.
Acid etching starts a process of hydrogen embrittlement in the treated metal. So this visual observation is not very scientific. Going to try some more, more bravely.
Other possibles: Navel Jelly is p. acid. Dupont has a product: Quick Prep. Sherman Williams: Metal Prep (auto paint supply shops).
Hydrochloric acid and Oxalic acid will do a quick removal but are too aggressive and not as 'selective' as phosphoric acid appears to be.
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