Can I use this resin as a top coat over previously painted wooden floor? 

Yes it will work on a dry painted floor. Ideally give the floor a light key and ensure the floor is free of dirt and dust to maximise adhesion.

We would recommend you test a small area first to ensure the paint itself is not damaged as some paints will be damaged under the resin.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank you