Second-Coat Adhesion Problems With Varathane Oil-Based Urethane Floor Finish

By Bruce W. Maki, Editor

When I applied the second coat of urethane on a floor refinishing project, the wool applicator seemed to drag a bit, and the finish seemed to have more "brush" strokes than normal. Later that day, when the second coat had dried, I noticed that the finish was terribly rough.

Upon closer inspection I could see a wrinkled appearance to the surface of the urethane. Almost the entire 400 square feet of hardwood floor had this wrinkled texture.

Note the cloudy or milky appearance. This is an indicator of urethane that has failed to adhere to the surface below.

Some of the wrinkled areas had considerable cloudiness.

I called the customer service number listed on the can (1-800-635-3286) and spoke with a man who was immediately familiar with this wrinkled finish. He explained that the humid weather we've been having was the root cause of these wrinkles.

The weather was cool but quite humid when I applied the first coat of Varathane. The next afternoon I scuff-sanded the first coat and applied a second coat. Apparently the first coat had not fully dried and was only dry on the surface. The Varathane tech rep said that in humid conditions it can take 2 or 3 times as long to fully dry.

Under normal humidity levels, Varathane will be fully dry within 12 hours. I didn't give the first coat any more than 18 hours to dry, when in reality 36 hours would have been better.

The Remedy:

I tried scraping the floor with a carbide paint scraper. The urethane scraped up easily. I guess it wasn't sticking terribly well.

After about 45 minutes of scraping, I had removed the entire second coat of urethane from an 11' x 12' room. Problem solved. I swept up the urethane shavings and dumped them in the kitchen trash can.

Then the story really gets interesting. It was about 1:00 in the afternoon when I finished scraping the first room. Around 7:00 pm the situation started to stink. There was a powerful odor coming from the kitchen trash can.    It turns out that the pile of scrapings had begun to spontaneously burn. The temperature was 360 degrees F on my infrared thermometer. Read More About Spontaneous Combustion With Urethane Scrapings and Dust.

After the floor was scraped, it had a dull appearance. You can see some spots that my scraper missed. It's easiest just to sand these down.

After another day of drying, (just to be absolutely sure) I sanded the floor with a random orbital sander and 120 grit sandpaper.

I didn't scrape the urethane from this narrow strip of floor. I decided to experiment on this useless part of the room. I used a pole sander on the back half, and a random orbital sander on the front half. I wanted to see how well the next coats of urethane adhered to the wrinkled second coat. It seemed to stick quite well, but I question the long-term adhesion.

This does make me wonder if scraping the bad second coat was really necessary. Of course, I did this sanding experiment the day after I scraped the first room, so everything had time to cure better.

The random orbital sanding took a long time to remove the visible whitish/cloudy spots... probably more time than scraping and then sanding.

The hand-sanded section is the real experiment, because most of the bad second coat stayed on.

No Wrinkles: After the real second coat, the finish looked normal and smooth.

Then I continued and applied a third and fourth coat.

Click here to read more about applying urethane.

Copyright © 2006  HammerZone.com

Written September 6, 2006