im new here, but i do have some experience with cast aluminum prep and finishing,
i just today used a rust oleum spray on my dolphin striker and mast base,
the BEST way to achieve a powdercoated, durable, HARD finish is to first wire brush the part with a fine wire wheel, and completely strip it of all of the corrosion that alum naturally accumulates, you want a shiny, brushed surface, then wash in HOT water with a degreaser (such as dish soap, its stronger stuff than most believe, it will outdo alot of solovents given due time) after that, use a hot water rinse, be sure to wear latex gloves before removing from the water, as at this point you do not want your skin to come in contact with the aluminum, as alum after being brushed tends to "absorb" hand oils and easily become contaminated, then hang from a post in by a very small gague wire in a well ventilated area, use rustoleum clean/very lightly rusted spray primer, do 2-3 VERY LIGHT coats, the first coat should barely dust the aluminum, second should moistly cover, third should fully cover and shuld be the thickest, allowing 5 min for the first coat to dry, and 10 min drying between the last two coats, let dry for an hour in a warm dry environment, using warm DRY (moist heat will destroy your parts finish) heat, not too hot will speed the drying process and will actually harden the primer, after an hour rehang part by its wire and use your finish color of choice, i love using metallic colors, as after about 3 coats (following same process as primer) and proper drying time, it looks like metal flake powdercoating and all the tiny brush marks that "ruined" the look of your part and the pitting from corrosion, dissapears from sight,
from here you can either chose to go with this, let it stand at least a week before actually using the part, you can go ahead and install just keep dry a few days, it will count in the long run, this will last you a year, or two at the most looking half decent.
OR you can continue to put a clear coat on.
theres many acryllic and epoxy spray clear coats available today for a low price, a clear coat will not only add one more layer of abbraision and impact protection, but in some cases will protect part from uv damage such as fading, and will make your part last much longer, clear coats should be applied in a series of 8-10 very light coats, with 5-10 min between coats, pending on your choice of coating
i try to avoid laquer simply because ive had experience with it blemishing heavily under salt water conditions.
if you use a metallic, you can get a great metal flake powdercoated look, or use a flat black and achieve a great flat powder coat, rust oleum also makes an awesome paint called "hammered" now that creates that rock hard, hammered tool box look on virtually anything thats primed well, ive done skateboards, street luges, boat parts, car parts, tools in this finish and its extremely durable and looks crazy cool.
this may all sound confusing, but take the time to do it right, wait for a warm but not too hot day, make sure its not humid at all out, wind can work to your advantage if your outside, but if your in a dusty area, work in a well ventilated area out of the wind like a shed with exhaust fan running and windows open, itl matter when you find 100's of dust particles jutting out of your beutifull finished parts.
i usually move parts indoors into warm dry basement and hang there overnight, once paint has cured for about 10 min, its basicly odorless so the wife wont get on you about it.
sorry, im rambling
any other questions about metal finishing etc etc, let me know,
im planning on doing the rest of my boat in the same methods/colors as this, all the frame, mast and cast alum parts, including boom, will look snazzy
peace
jason
