If your going to paint it first you will have to get it
clean. Its been exposed to oil and aluminum is
somewhat penurious. One of these water based
grease removers is probably the best choice.
Let it soak in a 50/50 mix for awhile then rinse
it off well. The metal etching primer that Garry
mentioned is a necessity if you want the paint to
stick for any amount of time. You should understand
that the paint on a spinning prop is being exposed
to roughly the same strain as if it was being sprayed
with a high pressure washer. Any fracture or week
point will lead to the paint pealing off epically at
the outer edges of the prop. When I paint a prop
I use a metallic (silver – gold – copper). No
primer. I just clean it well and spray it on. The
metallic’s touch up well and with out the primer
you can easily strip the paint off (lacquer thinner) and
redo it for a show or just to hang on the wall till
spring.