There are several ways to painting aluminum boat window frames and many opinions on the subject, but here is a way that works and looks great 5 years later so lets' get started.
Having aluminum window frames on your boat that have already been painted, but the paint is mostly gone from weather and/or waxing (cleaner waxes actually takes off layers thru the years), looking gray with parts that still have some paint on, are the first type we'll redo. Around either side of the framing, put blue masking tape (doubled or tripled for protection) so not to scratch the window or gelcoat, so make it as wide as you feel comfortable. Take a drill with a wire brush attachment (2 hollow in the middle for easy use) and go over the framing lightly. This will take off any corrosion and small dimples you can go over the painted part also, making everything nice and smooth with no high points. Make sure to replace or add to any masking tape that ripped or came off. Go over the framing with a rag to take off dust and residue. Take another rag with a touch of acetone and go over again. Wipe with a clean rag and making sure it is dry take 220 grit emory paper and lighly go over the framing, looking for discrepancies that need to be lighly gone over again. Finish up using the drill with the wire brush attachment. Use rag to clean and go thru the same procedure as above with the acetone. The boat framing should be smooth, some areas may still have the paint on, and the aluminum is bright and clean. You need it to be clean before painting boat aluminum framing!
Now put paper on and around the boat window, because we will be spray painting it. Put the paper far enough around to take care of any over spray.and of course, do it on a calm, dry, warm day. I use high heat engine spray paint (spray can) that you can find in many Marine Stores or Auto parts stores. I think it has better adhesion properties, and the Boat frame. Since it is aluminum, does get mighty hot in the direct sun.
There is a little secret to spray painting from a spray can. Since you can't put in all those extra solvents that professional painters put in their spray containers, you can do a simple thing. For painting aluminum boat framing, you should get the surface you're painting very warm, I mean Very warm! You can accomplish this by putting what you are spray painting directly in the sun that'll do it. Start painting boat aluminum framing by giving a very light coat. It doesn't matter if you cover everything a matter of fact, don't just a quick light coating. You will be able to see right away if you didn't clean the surface well enough. If not get the acetone rag and take the paint off and repeat above. But I think you did a good job so we'll move on. After that light coat, give it a couple of minutes a repeat with another light coat. You will notice the paint is starting to cover some, not much, but some. Because the Boat aluminum frame is so warm, the paint sets up and dries quick! Keep repeating this process, and before you know it, you will have a beautifully painted boat aluminum frame like right from the factory.
I know some say to use a primer, but with that high heat engine paint, it must have other properties in it, because it lasts for many years. If your Aluminum Boat Framing has a lot of deep pits in it, then I would use a spray primer in a can, one that is made to go over rust. It is thick so it fills those pits and you can lightly sand it down to get off the excess. Then follow my spray painting method above and remember to have it in the sun to be very warm, no sun, use heat lamps Enjoy the New look of painting Boat aluminum window frames!