It's a pain.

If it is indeed painted, you're going to need heavy duty paint stripper like Jasco. The stuff is nasty and you need to do it outside with decent gloves. It will actually eat away at cheap rubber kitchen gloves.

I mounted my rim (no tire of course) on an old fork and stuffed the steerer tube in a car jack stand with the center section removed (kind of like a xmas tree stand but a lot sturdier). It makes it easier to spread this stuff around if the wheel is mounted/can be spun and swiveled around.

Just pour some Jasco/stripper out on an aluminum paint tray (not sure what the stuff will do to plastic) and use a paintbrush to brush it on. Leave it sit for awhile and it will start to bubble up. Then use a PLASTIC scraper to remove the paint.

Usually takes multiple coats of applying the stripper, waiting awhile, scraping, wiping down, reapply, etc..

Then use a something like an odorless mineral spirits to wipe it down to see if you missed any major spots.

At this point you have two options.

-Take it to a shop and have them pull the rim off and then have it powder coated/painted, reassembled.

-Cheaper way: Spray paint. In order to do a decent job, you need to take your time. Mask off EVERYTHING you don't want overspray on. Use good blue 3M painter's tape. That means every spoke nipple, etc..
If you want to be extra careful, clean with acetone or use a sticky painter's cloth. Forget the name but it's just a sticky cloth that removes dust/dirt.

Choose a standard primer...really doesn't matter. Any decent spray primer will do.

Then figure out what kind of finish...gloss, semi-gloss, flat, satin, etc.. I prefer Satin because it hides mistakes a little better than glossy and seems to hold up well. It's also easier to wipe down than matte/flat which seems to easily get stained with oils, grease, etc.. I used Rustoleum Satin since I had good success in the past.

The trick again is to use a stand so you can spin the wheel while spraying. I used an old camera tripod (heavy duty Bogen) and built a wood section to mount the wheel on so it could spin/pivot freely.
Then just spin the wheel, spray off to the side first and then move onto the wheel for a few seconds, let dry. Lots of light coats is better than heavy coats. Also test the sprayer to see how far back you want to be. The spinning wheel method works great because the whole wheel gets covered and you never have to worry about one section of the wheel getting too much paint or the paint can being held in one spot for too long.

I likely did around 4-5 light coats of primer (some were super light to hit just one missed section) and twice as many with paint. LIGHT COATS. Can't express this enough.
You can use clear coat if you want. I didn't but it might better too.

Try to find a contained area with minimal breeze. I used a narrow section at the back of my house and draped a cloth behind everything for overspray.

It's a lot of work but cheap...Jasco paint stripper (small can) is like $8...think I bought one can of primer and 2-3 cans of paint. Whole deal was under $25 but I already had the 3M blue tape/cheap brush/plastic scraper.

I have a friend that owns a car paint shop and he was actually impressed how well they came out for a spray can.