I’ve achieved good results by starting out with the 4-inch 180 grit abrasive balls from harbor freight followed by hand sanding. The trick is full strokes all the way down the rail in one direction only. Never stop in the middle or work back and forth. Keep your pressure nice and even. Use a sanding block for the hand sanding part. After the abrasive ball, I use 180 grit sandpaper and step up grit by grit to at least 1200.