I've powder coated my 1991. Use prismatic powder silver with a prismatic clear coat https://www.prismaticpowders.com. For the sealant, I used several tubes of GE advance silicone (exterior) found in home maintenance stores. The Harley Davidson people use it on their spoke rims. I then used balance beads versus external weights. All works like a charm.

As for the "they flex" issue. modern powder coats will flex within the limits of the rim without issue. I did mine two years ago, 2K miles later, they still look new.
[Linked Image]

[Linked Image]

I would suggest that you hand prep each rim with 600/800 sandpaper and a good sandblasting. They need to be as clean as possible. Tape off the hubs to prevent sand from getting in them (and then clean and re-grease when you get them back).

If I remember correctly, the color was British Silver Swatch /Item: PMB-0850S

Big debate was if the rim was "tubeless" or not. I read that since about 1980, they were all tubeless. When you pull the tyre, you should see a ridge that retains the tyre on the rim. The heat of the oven will destroy the seal so you will either have to use a tape or silicone. The GE has held up just fine. this is a photo before they were sandblasted. both a tape and silicone were used before me! [Linked Image]