I assumed that the aluminum arm was simply clear coated at the factory, but my efforts with paint stripper didn't faze the finish.
I ended up sanding the affected areas with a metal prep pad on the die grinder down to the aluminum. After experimenting with various types and grits of wet/dry sandpaper, I eventually got the finish acceptably close to original.
Because they're now bare aluminum, the refinished areas will require occasional future maintenance to keep their sheen.
With the area cleaned and detailed, the wheel and brake can now be remounted, and project 19 will be back on its own two feet.