Tell the guy at Lowe's to stick with painting barns. What kind of finish are you trying to achieve? The easiest method with poly is to apply with a pad, sand with 220 grit between coats to keep it smooth and then kiss it with a final coat sprayed on. Start by sanding down what you have with 120 to get a decently smooth base.More advanced techniques involve wet sanding with superfine grits and then buffing with different grits of rubbing/polishing compounds.