The best applicator for applying oil-based polyurethane to wood floors is a lambs wool applicator. This accessory is effective in spreading the polyurethane evenly across the surface, much more so than using a paint roller, although a roller can work with water-based products. Before using a lambs wool applicator, remove hair, lint, and other debris from the wool’s surface. An easy way to do this is to cover the surface with painter’s tape or anything similar and then ripping it off. All the loose debris will stick to the tape, so it doesn’t end up on your floor during application. For corners, skirtings, and other nooks and crannies, use a natural bristle brush. It’s up to you whether you find a 3” or a 5” brush most comfortable to work with. Just use a finishing brush and not a regular paint brush.