Mr. Bow Saw
Addicted to ArboristSite
Sand/scrub it off with scotchbrite pads or sandpaper and light oil. I use a 3/1 mixture of Kerosene and ATF. The oil protects the newly exposed magnesium from more oxidization. If you plan on repainting the mag, sand/scrub with dry pads/paper, flush with stripper and then prime immediately. Otherwise, the freshly exposed mag will start corroding again.
Don't use steel brushes or pads/sandpaper that has been used on steel. You can embed steel/iron oxide in the magnesium, and that will continue the corrosion.
I have use a wire wheel on a lot parts for the saws I have restored and have not seen any white death come back or any start up.
I do hand sand the parts before painting them.