Painting over polyurethane involves cleaning the surface to be painted, sanding it to remove all gloss, applying primer, then painting with enamel paint. Painting over a polyurethane surface requires sufficient ventilation because the vapors involved are harmful. Painters should wear a respirator when doing this task.
The first step in painting over polyurethane is to clean the surface to be painted with a trisodium phosphate solution. All dust and dirt should be removed, and all stains should be scrubbed away. Remove and clean any hardware on the door.
After the cleaned door has dried, sand it until smooth. It isn't possible to paint over a polyurethane surface without roughing it up because the new layer of paint can't stick to the surface and is likely to peel off. The entire existing stain does not need to be sanded away, but the glossiness of the surface must be removed. Liquid sandpaper or paint de-glosser can be used instead of actual sandpaper. After sanding, the surface should be cleaned again to remove all dust.
After sanding, prime the surface with a primer appropriate to the type of paint being used. Let the primer dry overnight, resand any uneven areas, then prime the surface again.
After the primer has dried, enamal paint can be applied. Each layer of paint should be allowed to dry before adding another layer.