Just a couple of points...
1) call ZephyrWorks http://www.zephyrwerks.com
They can help walk you thru the process of measuring the sheaves and then custom make Delrin ones for you. Figure about $30 for a custom sheave, a little less if it's one they have on the shelf.
2) I would never recommend painting an aluminium mast. It doesn't add much in the way of protection, and is very difficult to do well and once you start painting you either have to keep redoing it forever or strip it all the way back to aluminium. Basically it's a lot of work for very little benefit.
3) if you are going to cover any self protecting metal (stainless, aluminium, etc) be very careful that the cover doesn't hold water
next to the metal. Or if it does that it will dry quickly. Over the long term stagnent water
held against aluminium can cause poltice corrosion
4) the protective coating is likely just a clear coat added for cosmetic purposes. I would just remove it, make sure there is no corrosion underneath it then let it be.