I have some exterior doors with glass that his held in place by a sandwich of plastic(?) grilles. The grilles have turned yellow and look bad. What would be the best way to paint these to look and stay white for a while?
If the surface is relatively smooth (not peeling or cracking), I would just clean it off, tape up the glass, and then paint it with a coat or two of an exterior-grade primer.
If you just want flat white, you could even wait until the primer dries and see how you like the look. If you like it, leave it primed. If you want something fancier, a high-gloss, exterior-grade white paint ought to do the trick.
(Foam brushes are sometimes a nice way to go when painting super-smooth surfaces like plastic.)