Sanding off a drip, run or sag in varnish is a pain. It takes forever and you’ll always see evidence of the mistake after the piece is finished. A better fix is to scrape off the drip with a super-sharp wide chisel. Hold the chisel at a low angle and don’t try to take it all off at once. Come at it from different angles. With a little practice, you’ll be able to remove the drip without damaging the previous coat of finish. When it’s gone, lightly sand the area and recoat.