MAGNESIUM ! Great lightweight metal .....but a real SOB to maintain ! I've studied up on this subject since I bought my burners about 6 years ago (and 3 different restroations of the same wheels !

) What happens is the metal "gasses off" over time when the climate changes too much (eg: you leave them in the garage), and then wheel will start to blister - crinkle - flake or peel. The wheels need to be COMPLETELY coated with a good primer and paint over the ENTIRE surface area including the inner barrel. Otherwise with the change in temperature, the uncoated areas will strat to pull the gasses through the easiest exit points - the face of the wheel. My best advise is the try to keep them in a stable environment if possible - I know easier said than done. I hope this helps.
Darrell