Why would a paint film over a varnish cause chipping?

Paint applied over varnish or some wood stains can sometimes cause chipping if knocked. Varnishes are very hard films and adhesion to these surfaces is difficult. Should this happen, you'll first need to thoroughly clean down the surfaces to remove all dirt, grease and surface contaminants. Then carefully scrape back poorly adhering or defective paint to a firm edge and rub down to 'feather' broken edges. If chipping is severe, the surface will need to be totally stripped back. Dust off prior to repainting.