Paint over varnished wood by either sanding and priming or deglossing the surface before applying the final coats of paint. Sanding and priming work best on large, flat areas of varnished wood, and this method tends to produce the best final results. Deglossing is not quite as effective as sanding and priming, but it is a much easier method and works even in tight spots or on intricate trim or molding.
In both methods, the primary goal is removing the glossy surface that results from varnishing. Paint does not adhere well to glossy surfaces, so bubbling, cracking and other surface problems affect paint applied to unprepared varnished surfaces.
Sand a flat varnished wood surface using 180-grit sandpaper only until the surface is no longer glossy. Providing a matte surface is the goal, and completely removing the entire varnish layer is not necessary. Vacuum all dust that remains after sanding, and use a tack cloth to remove any stubborn dust still adhering to the surface. Prepare the sanded surface for painting using a high-quality oil or water-based primer. After priming, the surface is ready for painting.
Use deglosser by soaking a rag in the deglosser and then applying the product to the varnished surface using a repeating circular motion. Once the soaked rag stops picking up varnish from the wood, apply a layer of deglosser to the surface using a paintbrush. After the deglosser dries, apply an oil or water-based paint; no priming is necessary.