Applying urethane floor finish. Detailed Article:

Refinishing Hardwood Floors
Part 3:
Applying Polyurethane

 
In This Article:

The sanding dust is vacuumed up. Four coats of urethane are applied with a lambs wool mop or wide paint brush, scuff-sanding the finish before recoating.

Related Articles:
Skill Level: 2-3 (Basic To Intermediate) Time Taken: About 10 Hours, Over Several Days

By , Editor

Start:

Previously, we sanded the floors with the Varathane ezV floor sander.

Read more about sanding in Part 2.

 

The floor looked completely different after sanding.

Before applying finish, I carefully vacuumed the floor.

 

Ventilation:

Getting rid of the fumes is critical. I always use some sort of fan in a window to blow the air out. If the weather permits, I'll leave the fan going for a couple of days.

Keeping the power cord above the floor is important. Read about a quick tip to keep cords off the floor.

 

Applying Urethane:

There are 3 important concepts to applying urethane floor finish:

  • It's important to maintain a wet edge. This means that you should apply urethane in a pattern that progresses across the room, like a wave travels across a pond.
  • Always back-brush. Back-brushing simply 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. You won't see a mark where the brush lifts off the surface.
  • It's best to apply urethane by brushing with the grain.

 

For the first coat, I used a "lambs wool" applicator to apply Varathane Oil-Based Satin floor finish.

(Normally I apply urethane in a more systematic, continuous pattern. I only did this for the photo.)

 

This is an organic vapor respirator which you can see I'm wearing in the photo above.

You need one of these. Years ago I would just "tough it out" and endure the fumes. That's not smart. I bought this mask when I started doing spray finishing with urethane, and now I use it for all kinds of smelly work.

It costs about $35 at Home Depot or Lowes, and the replaceable carbon filter cartridge lasts a long time. This is money well spent.

Organic vapor respirator which absorbs fumes.

 

In the upstairs bedroom, the only way I could maintain a wet edge was to brush across the grain.

So after applying a 3-foot wide strip, I used the mop like a push broom to "back-brush" the urethane.

The weather was cool and humid when I applied the first coat. I knew that the urethane would take longer to dry... no big deal. I finished up around 7:00 on a Saturday evening. Since the house kinda stunk, it was a good excuse to eat out. I figured that the second coat could be applied sometime the next afternoon.

The Next Day:

After the first coat had dried I scuff-sanded the floor with 120-grit sandpaper on a pole-sander.

 

There were some clusters of bubbles like these.

 

After scuff-sanding, the bubbles were mostly gone.

 

Even though about 18 hours had passed since I applied the urethane, I still got some urethane build-up on the sandpaper.

 

So I just stopped sanding every few minutes and used a knife to pick off the "scabs" of urethane.

When the urethane gums up the sandpaper, it's a sure sign that the urethane has not hardened completely. When tiny "rolls" of soft urethane appear instead of sanding dust, it's even worse.

But I got lots of sanding dust, and no little gummy rolls. I didn't see anything abnormal, so I went ahead and applied the second coat of Varathane.

I've had this happen before when finishing wood trim, and I've never had a problem with the second coat of urethane. But... it could've been a different brand of urethane.

 

We Have A Problem...

Wrinkled appearance to urethane finish after one day of drying. Even before the second coat had fully dried, I knew something was wrong. The Varathane was full of wrinkles.

After calling the Varathane customer service toll-free number (1-800-635-3286), I discovered that I had applied the second coat too early. The high humidity had slowed the drying process for the first coat. When I applied the second coat, there was still a chemical reaction occurring in the first layer, and it affected the adhesion of the second coat.

So I had to scrape off the second coat of urethane. Even though I had 400 square feet of flooring to scrape, it didn't take too long.

Read more about this problem...

Scraping defective urethane finish from hardwood floor.

 

Recoating Urethane:

Sanding urethane before recoating. After the second coat (i.e. the successful second coat), I used a random orbital sander with 150 grit sandpaper to scuff-sand the floor before applying the third coat.

 

It took about 20 minutes to sand each room. There is a lot more dust visible after sanding with a random orbital sander.

I swept the floor and disposed of the sweepings outdoors. Then I vacuumed the floor with a shop vac.

 

Removing urethane sanding dust before recoating. After vacuuming there was still some dust on the floor.

To pick up every last bit of dust, I dampened a paper towel with rubbing alcohol and wiped the floor.

I found that the easiest way to apply the rubbing alcohol is to use a small garden sprayer that can be pressurized with a pump.

 

Imperfections:

This picture was taken after the 3rd coat.

That series of spots visible in the reflected light is called "blush". Blush is a spot of irregular sheen (shininess) visible after the urethane has dried.

Preventing blush can be difficult. I believe this spot was caused by the heavy coat of urethane forming a small pool before it dried. Applying a uniform thin coat of urethane usually prevents blush problems.

I also understand that blush problems can be caused when an excessive amount of paint thinner is used to dilute the urethane.

There were several areas with blush spots. I wasn't concerned, since I was planning on applying a fourth coat. Applying another coat of urethane always fixes the blush problem.

 

Fine Grit Fiasco:

When I sanded the third coat, I tried using 220 grit sandpaper, figuring it might be better than 150.

220 seemed to work about the same, but the dust was so fine that it caked onto the sander and the dust collector cup. When I tried to vacuum up the dust, it just stuck to the floor. And when I used rubbing alcohol on a paper towel to wipe up the remaining dust, I got a muddy mess. I kept finding tiny chunks of dried urethane mud on the floor.

I think I'll keep using 150 or 180 grit sandpaper for scuff-sanding floors.

 

Last Coat:

I decided to try applying the fourth coat with a 4-inch brush. I was hoping that brushing the final coat would reduce the overlap marks that I got when I used the lambs wool applicator mop.

But I still got plenty of overlap marks.

 

Imperfect Finish:
Overlap Marks

I wish I could get a picture of overlap marks, but they're difficult to see... you can't see them unless you're looking at the right angle. Overlap marks are visible as strips of floor with a different amount of shine. You can only see these marks in the glare of light that is reflected off the floor. Since cameras record glaring light differently than the human eye, I could not get a decent picture of these overlap marks.

These marks happened wherever I picked up the brush or lambs wool mop at the end of a stroke. If I could drag the brush over the entire length of the floor in one continuous stroke, I would prevent overlap marks. It's possible to get full-length strokes using the mop: You apply the liquid and then smooth it out using long strokes from one wall to the other. But sometimes the geometry of the room prevents such full-length strokes, unless you mop across the grain.

Urethane can be brushed or mopped across the grain. If you're lucky the urethane will level out before it dries, making the brush strokes disappear. I cross-brushed a few small sections, but I could still see some phantom marks in the finish... not ripples in the surface but variations in sheen.

Perfection is hard to achieve. But the good news is: You don't need perfection. Few people will notice the irregularities in sheen, and after the rooms have been used a while, the floors get scuffed a bit and all these imperfections are replaced with small scratches.

Perfection can be achieved by spraying urethane onto a surface, but that's not done on floors. Spraying is suitable for furniture refinishing.

 

The last coat, while still wet.

 

Final Results:

This is the main floor bedroom after the 4th coat of urethane had dried. It looked much better than before.

 

The upstairs bedroom. Now the walls need repainting.

After the fourth coat, I let the urethane dry for 3 days before doing anything in the rooms. While the Varathane will be dry after a couple of hours, it takes about 72 hours to fully cure. 

 

More Imperfections:

In spite of my careful cleaning, there were still some flaws in the final finish, such as this strand of hair. (It sure didn't fall off my head.)

Sometimes small specks of foreign matter can be scraped off with a fingernail. Mostly these little bumps will just get rubbed off from normal wear and tear.

 

More Info:

Tools Used:

  • Lambs Wool Applicator Mop
  • 4" Synthetic Bristle Brush
  • 5" Random Orbital Sander
  • Wash Basin

Materials Used For 400 Square Feet Of Flooring:

  • Sandpaper
  • Rubbing Alcohol, 1 Quart
  • Urethane, 5 Gallons
  • Paint Thinner, 1 Gallon
Related Articles:

 

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Written September 6, 2006