Varnish gives wood vibrant, natural-looking colour, and it helps seal the wood for longer-lasting durability. However, if you're unhappy with the look of a varnished surface, a dark varnished finish is hard to cover up. Paint will go on smoothly over the varnish, but you'll soon find that unprimed varnish bleeds through even multiple layers of paint. To get lasting coverage without bleeding, prep the varnished surface appropriately.
Sand the varnished wood with 220-grit sandpaper. This helps remove the topmost layer of gloss and oil from the wood.
Clean the sanded surface with a trisodium phosphate cleaning product and a sponge or rag. TSP is good for removing dirt and sandpaper dust, but it also helps cut through oily residues commonly found on varnished wood.
Dry the surface with a towel.
Prime the varnished wood, using a stain-blocking primer and a paintbrush. It's important that the primer is labelled "stain-blocking;" otherwise, you could experience bleeding days, weeks or even months down the road. Wait for the primer to dry.
Paint the primed wood surface, using any latex or oil-base paint. A regular paintbrush is idea for small items, but a roller will cut down on painting time for large wooden items. When painting over primer, the base coat may provide enough coverage. If, however, you notice any bare patches, paint another coat after the first coat dries.