Finishing a hardwood floor with urethane.  Do-It-Yourself Hardwood Flooring:

Finishing A New Hardwood Floor With
Oil-Based Urethane


In This Article:

The dust is vacuumed off the sanded floor, the fine dust is picked up with alcohol and rags, and urethane is applied.

Related Articles:

Skill Level: 2-3 (Basic to Moderate)

Time Taken: 4 hours over several days.

By , Editor

A Sanded But Uncoated Hardwood Floor:
Be Very Careful...

At the beginning: we had a sanded but uncoated white oak floor.

At this point, there were lots of hazards. We had to keep the door closed to keep out the dogs. Some dogs drool a lot, which could leave spots in the wood. And it would be easy to leave a black streak from a sneaker.


Before the main cleaning, I used a whisk broom to brush the sanding dust from the walls.


I vacuumed the floor with a shop vac.


To prevent the vacuum nozzle from marking the floor, I covered the face with masking tape.


Since the vacuum's wheels also tend to leave black streaks, I put the shop vac in a shallow cardboard box.


Black-soled shoes can leave marks on the floor, so I wore a new pair of slippers (about $8 at K-mart). 

In fact, this is a good excuse to buy a new pair of shoes, since new shoes aren't going to have any dirt or grit embedded in the soles.


The Final Cleaning:

After the floor had been vacuumed thoroughly, I used rubbing alcohol to wipe up the remaining dust. I poured the rubbing alcohol into a small pump-up garden sprayer, and wiped the wet floor with a paper towel.


Cleaning dust from a wood floor before applying urethane floor finish.

It was pretty simple: just spray the floor (since the garden sprayer is pressurized by it's built-in pump, I only had to pull the trigger) and wipe off the rubbing alcohol.

I went through a quart of rubbing alcohol in a few minutes. The smell of isopropyl alcohol became rather strong.

On one hand, ventilation is a good thing, but on the other hand, leaving a window open could bring dust into the room. I decided to leave the window open just slightly.

By the time I got to the end, and grabbed the camera, the floor was almost dry. I went through about half a dozen paper towels.

The key is to wipe only once with any face of the folded towel. By turning the towel constantly the old dust won't fall back onto the floor.


Applying The Urethane Floor Finish:

The basic tools: a new plastic dishpan, a 4 inch exterior stain brush (which is rather soft and works well for this job), and the urethane.

We've been using Varathane brand of urethane floor finish. We always use satin. Semi-gloss is too shiny, in my opinion, and gloss is right out.


This is just like painting. Dip the brush, tap the sides of the dishpan to remove excess, and apply.

Applying polyurethane with a wide paint brush.

As with painting, there are some techniques that should be followed, which not everybody knows. In the above photo I was working from left to right. When I first apply some urethane, I drag the brush any ol' direction.

But after I have swabbed a few brush-loads of finish onto the floor, I back-brush the urethane. This means that I lightly drag the brush from right to left. In other words, I drag the brush from the leading edge back into the finish.

If you stick the brush into the field of wet finish and drag it, you will see a very obvious mark where the brush first made contact. But if you start at a dry point and drag into the wet area and then lift the brush off gently, there won't be much of a visible mark.

The same technique applies to any aspect of painting with a brush.

After a few minutes I had made major progress. I covered a swath about 16 inches wide, working from left to right. Since I had marked the stud locations on the walls earlier, I used these marks as a guide.


In about 45 minutes I was done with the first coat.

The next morning the floor was dry but still a little soft. The first coat of urethane always raises the wood grain, so the high points must be removed with a fine sanding. 


Note the dull spots. The first coat often does not absorb uniformly. Or maybe I wasn't too careful with the paint brush.

Varathane recommends 3 or 4 coats for new wood floors.

There are some interesting rules to follow (i.e. the instructions on the can):

The Varathane instructions say:

  • Allow 4 hours to dry.
  • Recoat. Reapply only when previous coats have dried clear and feel hard (no longer tacky to touch). If any coat has dried more than 12 hours, lightly sand before recoating.

This means that another coat can be applied if the previous coat has dried and 12 hours have not passed, and sanding is not necessary. But don't bet on it.


I lightly sanded the first coat with 150 grit paper on a random orbit sander.

This procedure took about 30 minutes for 140 square feet of floor.

Scuff-sanding urethane floor finish between coats.


The Wave Zone:

I mentioned this problem in the article on floor sanding. In the middle of the room, where the drum sander was stopped to reverse direction, a series of slight waves formed, which appear in the photo as vertical whitish bands. In spite of numerous passes of the random orbit sander, these humps were not completely eliminated.


I can assure you, they were much worse before the random orbit sanding. The point is... slight humps that can barely be felt with your hand will become much more obvious when urethane is applied. They won't be as visible as the photo above, however. The sanding highlights the humps but not the valleys.

Another shot that shows the "mogul field" in the middle of the room.

This is the biggest disappointment of using a drum sander. Luckily, you have to get the light at the right angle to see these waves. The first hardwood floor we installed had much worse waviness.


Before the second coat I filled all the knot holes with a Minwax Blend-Fill wax pencil.


The excess wax was scraped off with a metal object (in this case, a paint can opener, which is like a wide screwdriver on one end). A putty knife would work just as well.


The Second Coat Of Urethane:

For the second coat I thought I would try a more expensive brush. This 3 inch sash brush by Purdy costs over $20. The bristles are quite fine and are good for oil-based paint.

But it didn't work nearly as well as the wider brush that seemed to have lesser-quality bristles.

I used the 3" sash brush for the second coat and it took a little over an hour. Since I couldn't easily see where the wet urethane was, I had to constantly examine the surface to look for the shiny areas and make sure I didn't miss anything.

A Note On Brushes:

There are other options for applying urethane to a floor, such as special broom-like applicators with lambs wool or mohair fibers. These are wider than my 4" brush and would certainly be useful if a larger area was to be covered.


The Third Time Is NOT A Charm:

After the second coat had dried, but before 12 hours had elapsed, I applied a third coat. I test-sanded a little patch in a corner. The urethane gummed up the paper a bit, so I decided that it was too early to sand. 

So I applied the third coat without sanding the second coat, which the instructions imply can be done.

It took f-o-r-e-v-e-r. Actually it took just over two hours. I believe I would have saved time if I had waited for the second coat to dry and sanded it quickly. 

When I dipped the brush and began applying a load of urethane, it seemed nice and fluid. But a couple of seconds later the liquid would suddenly feel really gooey and heavy. I suspect that the new urethane was slightly dissolving the previous coat and making  a more viscous liquid. The brush strokes didn't smooth out well at all. It was a fiasco.

My suggestion is: let Varathane dry for at least 12 hours, sand it, and then recoat.


Four Coats Of Floor Finish:

This time I let the previous coat dry for over 12 hours.

Before the fourth coat I went over the entire floor with fine steel wool.

For most wood finishing projects, I prefer steel wool instead of sandpaper because sandpaper often gums up if a small area of urethane has not fully hardened. Scuff-sanding with steel wool is much slower than sandpaper, but steel wool follows the contour of the wood better.


I know that steel wool is working properly when I can shake it and a bunch of whitish-colored dust falls out.


Update - 2005:

Since writing this article in 2001 I have covered a couple of other floors with urethane, and I have discovered a very helpful protective device: the organic vapor respirator. I bought a 3M organic vapor respirator at Home Depot for about $35, and it is worth every cent. This mask covers your mouth and nose, and has cartridges that contain activated charcoal which absorb just about every type of chemical vapor.  In this case "organic" refers to organic chemicals, those materials that contain carbon, which is darned near everything made from petroleum products, which is darned near everything.

Anyway, while wearing that respirator, I can't smell ANYTHING. Not even a hint of odor from the urethane.


The Results Are In:

And we love it. Even in the less-warm-looking natural daylight, it looks good.


The closet.


The back part of the room.

Those unfinished vertical strips in the corners are plumbing access areas, which will be covered by built-in bookshelves, as soon as I get around to it. 

After the floor had dried for a day, I carefully set some tarps on the floor and gave the walls a second coat of paint. Installing the hardwood and the sanding caused a lot of marks on the walls. When the paint was dry the room had all surfaces fresh and new.

All that remains in this room remodel project is to install the trim and some other finish carpentry. 


When the last coat of urethane is done, a certain euphoria comes over me. Oh... maybe that's from breathing solvent fumes!  For some reason I always forget how difficult all that sanding was. When I see what an awesome floor I have created, I am tempted to say "sure, I'll do another".

I guess the bottom line is: the results are definitely worth all the hard work.


Tools Used:

  • Random Orbit Sander
  • 4" Paint Brush
  • Plastic Dishpan
  • Garden Sprayer
  • Box Fan (For Exhaust)

Materials Used:

  • Urethane, Satin
  • Rubbing Alcohol
  • Paper Towels


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Copyright © 2001, 2005

Written September 9, 2001
Revised January 11, 2005