If you are not changing the color scuff the finish with some scotch brite red pads. Assuming it is a veneer like oak wiping will be fine. Tack cloths work best. Doesn't matter if you are spraying or brushing on a top coat just let it dry for 24 hours. After the first coat dries scuff the finish with some 320 dry paper, wipe off, and repeat. If you want a nice shinny high gloss repeat about 3 times and keep reading.
Once you have some layers of finish on there go get some wet/dry paper (600-1000 grit) sand the finish until it is a dull flat finish.
Once flat grab a buffer and buff the finish out like you would a car. With the last two steps be careful not to burn thru the finish. Treat this as if you are buffing out a tank.