I have painted several, and here is how I do it:
1. Sand the top surface of the plate with 80 grit sandpaper
2. Spray thin coats of paint (usually 3-4), allowing to dry at least 15 minutes between coats.
3. Allow plate to dry for 24-48 hours before installing switches and stabilizers.
4. Profit?
I don't find the need for a clear coat. The keycaps should never hit the plate, as the switch bottoms out before the sides of the cap would touch.