Just a scuff is enough
Today's super-durable polyurethane finishes don't stick to themselves the way layers of shellac or lacquer bond with previous coats by partially dissolving them. That makes sanding between coats essential. By doing so, you remove dust nibs that become trapped in slow-drying, oil-based finishes. You also leave fine scratches that give the next coat of finish a grip on the one below. Scuff sanding is easy: Just make about four light, uniform passes over the finish using 280-grit abrasive. Check the paper frequently to avoid clumps–called "corns"–that can mar your finish. Use a flexible pad or your hand. A flat, rigid sanding block increases your risk of sanding through the finish at the edges and on uneven spots.