I painted my alternator without taking it apart. I paid to do it before (polish), it was a waste of money IMO.
Get some Scotch blue painter's tape... Get a flat headed screwdriver, and a pick that is 90 degrees. Basically, you are going to take your blue painter's tape, fold it over on itself so all the sticky sides are stuck together. You then slide it into an opening on the alternator, and wiggle it around the case inside. Keep your lengths of tape LONG, 6-8" so you do not lose any small pieces inside causing a problem or potential fire when in use. After a while, you should have blue tape visible through every cut out. The pick is used to stab the tape and pull it through/move it around inside. Also aids with removal...
I painted mine with VHT silver high-heat header paint. It's a dull finish, gloss on that part will turn dull after a few weeks anyways, don't waste your time. Once the car heats up a few times, it'll adhere/bake-on a bit better. I am sure it'll flake/chip if you hit the part afterwards, so, use caution when / if you complete this project. Mine has been done for a year, still looks brand new, no more corrosion/flaky white stuff when sitting in the winter.