Best Varnish For Exterior Wood Doors . Apply 2 coats of altex timbercote marine varnish. This door has only one panel, at the bottom. Need to refinish a varnished, wooden exterior door which varnish is from www.reddit.com Top 10 reviews on the best spar varnish for exterior door 1. Deft defthane interior exterior clear polyurethane gloss. At this stage you “lock in” the raw wood substrate with the finish and then build on that initial layer with the finish coats that follow.