Are you married to painting them? My cars are missing these pieces, so I've tried to educate myself about them since I really don't want chrome pieces on the car, which I've seen occasionally. My concern with paint is that you'd have to make sure it's only on the outside of the parts since, if it gets onto the gripping bits, it may compromise the holding of the parts to the window.
Have you looked into getting the parts anodized? I think that was the original finish.
Good luck