How to Apply Polyurethane

Polyurethane is a type of synthetic varnish used for finishing and sealing wood. In interior applications such as finishing floors and wood furniture, polyurethane is harder and more resistant to abrasion than its naturally-derived counterparts. It is also cheap, easy to use, and available in a range of different finishes from glossy to matte. Learning how to apply polyurethane in a wood finishing project is a straightforward matter requiring only a few tools.

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Steps

    1
    Position your work for applying the polyurethane. You will need to work in a well-ventilated area, because polyurethane produces harsh fumes. It is also important to have either old towels or scrap newspaper on hand to catch spills; polyurethane is designed to bond to surfaces, and is therefore very difficult to clean up.
    2
    Sand your wood before applying any finish coats. Whether your wood is unfinished or has an old finish on it, go over the whole piece with fine-grit sandpaper. Sanding removes dust and debris, and also improves the polyurethane's adhesion.
    3
    Wipe the piece down with a clean tack cloth. After sanding the wood, wipe it down with a clean microfiber tack cloth. This removes any dust or small wood particles left behind by the sandpaper. Generally, sanding should always be accompanied by a cleaning with a tack cloth.
    4
    Stir the polyurethane in the can. Open the can of polyurethane and stir it gently but thoroughly with a paint stir stick. If you have purchased a semi-gloss, satin or matte finish, make sure to stir the flattening agent up from the bottom, or the finish will be too glossy. Avoid stirring too vigorously, as this could introduce air bubbles into the product that will mar the finish.
    5
    Load your brush with polyurethane. Dip your brush into the can of polyurethane, and then wipe off any excess on the can's rim. Polyurethane is best applied with a stiff, China bristle brush, but can also be applied with a foam brush.
    6
    Apply the first coat of polyurethane. Use your brush to apply the finish in long, slow, broad strokes along the grain of the wood. Spread the finish thinly to avoid drips. Once the polyurethane has begun to dry, be careful not to brush over it. Applying fresh polyurethane over a partially dry patch will reduce the sheen of the finish, resulting in a more matte look than intended.
    7
    Allow the first coat of polyurethane to dry. Polyurethane dries fairly slowly, so it is best to wait a full 24 hours before applying the next coat.
    8
    Sand the wood and apply the next coat. After the first coat is dry, go over the wood again with sandpaper and a tack cloth. Apply the second coat of polyurethane just as you did the first. If desired, a third coat be be applied following the same method.

 Tips

    To speed total drying time, you can replace the first coat of polyurethane with a fast-drying sealer such as Zinsser's Seal Coat.

 Warnings

    When using polyurethane for outdoor applications, be careful to buy a product advertised for outdoor use. Regular polyurethane will break down quickly when exposed to the ultraviolet light of the sun.
    Polyurethane should not be applied over a shellac finish, as it will not adhere well and will peel away.

Things You'll Need

    Towels
    Wood
    Fine-grit sandpaper
    Tack cloth
    Polyurethane
    Paint stir stick
    Paint brush