Applying Polyurethane to Hardwood Flooring

by DoItYourself Staff

Although applying polyurethane to a hardwood floor can be a difficult and frustrating challenge if you're inexperienced with this product, with a set of good instructions and the right tools and materials, you should be able to successfully complete the process.

The amount of polyurethane you'll need will depend on the area of the surface you'll want to cover. You'll find this coverage information on the product you choose. As for tools, here is a list of tools and materials you'll need to collect before you begin applying your product:

Orbital Floor sander
Shop Vacuum
Exhaust Fan
4" Paint Brush
Lambs Wool Applicator
Organic vapor Respirator
Mop
Pole Sander
120 grit Sandpaper
150 grit sandpaper
Knife (or putty knife)
Paper towel
Rubbing alcohol

Preparation
Before applying your product, you'll need to prepare. Here are 5 things you should do:

1. Urethane fumes
Get rid of them. This is critical. Use a window fan when applying the product.

2. Floor preparation
To avoid getting unwanted urethane on your baseboard you will need to mask the baseboard or remove it while applying the coating. Remove any old floor coating by sanding it with an orbital floor sander. If the floor is new you may need to scuff sand it before applying your product.

3. Vacuuming
Vacuum all dust on the floor remaining from sanding.

4. Respirator
Wear an organic vapor respirator while in the room where the urethane is being applied—and even for a day or two afterwards if you're going to spend extended time in the room.

5. Keeping the extension cord off the floor
When applying your product and using a fan it is important that you keep the fan's AC cord off the floor you are coating. A good way to do this is to cut some split-flex tubing into short pieces and screw them into wall studs. Then insert the extension cord into the slit in the tubing.

There are 3 important things you'll need to know about application.

Maintain a wet edge. You can do this by applying the urethane in a pattern that progresses across the room
Always back-brush. Back-brushing means moving the brush from the wet edge back into the wet finish. If you stab the brush into the finish and then move it, you'll see a mark where the brush landed.
It's best to apply urethane by brushing with the grain.

Applying the urethane
Apply with a lambs wool applicator. After applying a 3-foot wide strip, you can use a mop like a push broom to back-brush the urethane.

After the each of the first three coats has been applied and has dried, scuff-sand the floor with 120-grit sandpaper on a pole-sander, then apply the next coat.

After the 4th and final coat, allow the urethane to dry completely. Typically, depending on the temperature and humidity, it takes about 72 hours for the urethane to fully cure.

You might see tiny—almost imperceptible—brush marks where your brush first touches and then is lifted from the floor. It is nearly impossible to apply urethane without leaving these marks, unless you can find a way to brush completely across the room in one stroke. But these tiny marks will rarely be noticed.

When finished, if you follow the rules above, you should have a nice looking finish that will protect the floor from moisture and from becoming dull.