Things You'll Need
Foam or lamb's wool applicator
Everything's going smoothly when you're finishing or refinishing your hardwood floor until you get to the polyurethane process and suddenly there are brush marks streaked all over the floor. You should never use a regular paintbrush for applying polyurethane. Hardware stores sell special foam and lamb's wool applicators designed specifically for this task. The wool and foam applicators leave the polyurethane looking smooth. If you've applied only a layer or two of polyurethane with a brush, you can salvage the project by painting more polyurethane over it with the correct tool.
Sand the floor lightly by hand with 400-grit sandpaper. This will help smooth out the brush marks while scuffing the polyurethane, which helps the subsequent coats adhere to it.
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Wipe the entire floor with a tack cloth to remove any dust from the sanding process. Apply the next layer of polyurethane with a foam or lamb's wool applicator. Do not use a brush when applying polyurethane unless you must for corners and edges.
Smooth out fresh, wet brush marks with your foam applicator. Coat the entire floor with polyurethane and allow it to dry completely. Scuff the floor again and wipe up the dust with the tack cloth.
Repeat the entire process until you can no longer see the hardened brush marks. Floors typically need at least two coats of polyurethane, but many professionals put as many as five or more, especially if the floor will be seeing heavy traffic.
Do not scuff the floor before applying the final coat of polyurethane.
Foam applicators come in many sizes. Buy a small one for applying polyurethane in corners and along edges of your floor.
Wear gloves, goggles and a respirator when working with polyurethane.
Open all windows to the room to allow for good air circulation.