You need to degrease the surface, then spray with Epifanes Washprimer AQ adhesive primer. It can then be sprayed with two part paint or coated with one part. It is important that the AQ is sprayed on to get a smooth finish. It can be used directly on anodised aluminium.
Iain
If it's removable, you'll get a better, more hard wearing and more professioanl looking job if you have it powder coated. Look in yellow Pages. Well worth the extra pennies.
on my mast and also on my catamaran drive leg, i fist coated with the epiphanes primer above (no reason to think that a 2 part primer wouldnt work just as well) then undercoated with epoxy and finally glossed with 2 pack poly. both the epoxy and poly were hempels paints intended for general industrial use on metal fabrications and purchased out of yellow pages at far less than the chandlers. the paints were spray-able, gave an excellent finish, and even stay on my propellor! the poly is as hard as h....
The most important step you must take prior to applying any paint to an aluminium surface is to etch it, this can be done with acid or etch primers, with this completed you can apply almost any paint, however a good Polyurethane would do an excellent job, check with your local paint supplier as you may need a coat of primer after etching.
Avaniceday Old Salt Oz.