Varnish gives wood a slick, glossy surface that's easy to clean. If you want to paint varnished wood, you'll find that this slick surface tends to reject paint. Latex or oil-base paint will coat in uneven, streaky layers for a sloppy finish. Fortunately, your local hardware store sells all the products you need to fix this issue. With light sanding and the right primer, you can paint right over a varnished piece of wood.
Sand the woodwork with 220-grit sandpaper. Sanding roughs up the varnish for better priming results.
Wash the surface with a trisodium phosphate (TSP) solution. TSP cuts through the grease and old deposits that can inhibit primer adhesion. Many pieces of varnished woodwork also feature stain or natural wood tannins. These oily substances are impossible to fully remove from the wood, but TSP will reduce the surface stain.
Dry the woodwork using a paper towel.
Use a paintbrush to apply stain-blocking primer to your varnished woodwork. Stain-blocking primer is a good idea even if the wood isn't visibly stained. Cedar, cypress, maple and some other types of wood naturally seep oils over time. These oils bleed through paint coatings if you don't use a stain-blocking primer. Wait for the primer to dry before proceeding.
Apply a coat of latex or oil-base paint to the woodwork, using a paintbrush. If you're using semigloss or high-gloss paint, use a foam brush to prevent unsightly brush strokes. For flat and eggshell finishes, you can use a regular paintbrush.
Paint the surface with a second layer of paint if necessary after the first coat dries.