How do prep a water pump before painting it? Do I mask off any part of the shaft before painting? Thanks!

No masking is needed. If your using a spray can get some Duplicolor or VHT engine enamel and some engine primer. Spray the part down with brake cleaner real well to get off all the oils.Works well,leaves no residue and is cheap. scuff it up a little,Prime it then paint. This is what i would do on a NEW or reman pump, that is clean, bare metal.Now if you want a really nice finish that will last years,the best way would to prep it like i said,then prime it with epoxy prime ,then single stage urethane in your choice of color.