Yes, DiamondFinish Clear can be applied over other coatings that are fully cured.
To prep that surface: Clean surface with KBS Klean diluted 10 parts warm water to 1 part KBS Klean. Scuff with 600-1200 grit sandpaper and repeat cleaning process. Rinse and allow to thoroughly dry.
Please be sure to read the complete directions for DiamondFinish Clear.