Once clayed, your paint will be smooth as glass. It will cause slight marring though so its best to machine polish after claying, then apply either sealant or wax protection.
Its fine to use clay on plastic and glass, how else would you do your bumpers? The key is to ensure that the surface is properly lubricated at all times, oooer missus
This is about the best guide going: http://www.polishedbliss.co.uk/acatalog/guides_clean.html
