Hi..
I put your question to an anodiser in Emsworth whilst collecting some work for my company on Friday. They routinely treat new masts and spars. They advise it is certainly possible to re-anodise an old mast but not easy and the results could be dissapointing. The mast would need to be stripped of all non aluminium fittings and fasteners. It would need to be caustic etch cleaned to strip the old surface. This would remove some material and slightly weaken the mast. The cosmetic appearance would likely be dull and possibly patchy. From my experience painting is not really viable either. Looks like your stuck with it. š„
Barryboy.