There’s a recent phenomenon that is perplexing hardwood floor installers throughout the industry. An adhesion problem presents itself when screening and recoating old oil-based polyurethane finishes. Some have dubbed this phenomenon of adhesion failure “Self Healing.”

City Floor Supply’s Mike Glavin recently witnessed this problem first hand.

Mike was contacted by a contractor who was working a jobsite that was about 4,000 square feet. The floor was lavish in its design. It was parquet with a large “medallion” in the center. The job required the contractor to screen and recoat the floor. Mike helped the contractor with a plan of attack so the recoat would go as smoothly as possible.

The floor the contractor was working on was roughly divided into three parts. Two large hallways, or wings, were on either side of the center of the floor. The finish that was present on the floor was an oil-based polyurethane. This finish had been applied to the floor almost two years previous to the screening and recoat.

The contractor thoroughly cleaned the floor and then screened the floor with new and sharp 150 grit sanding screens. The contractor used 10 screens on the project. The wings of the floor were immediately coated with the new oil-based polyurethane. The contractor and the crew left the jobsite without coating the middle of the floor. They came back after a few days and applied the finish to the middle area of the floor.

But then something completely out of the ordinary happened. The finish was not “sticking” to the floor and it scratched off easily. But this “sticking” issue only applied to the center of the floor. The two “wing” sections were fine.

So what happened during this seemingly normal recoat job?

For that answer we need to side step for a second.

On first glance they didn’t really do anything out of the ordinary or wrong. On finish jobs for brand new floors it’s typical, common even, to sand a floor and apply a coat of finish. Perhaps then the floor installers have to wait a week or two before they can get back into the jobsite and do a few more coats. This process is typical with water or oil-based finishes.

Another factor to consider is that if this ornate floor the contractor was working on had been installed with a water-based finish, then all the steps he took would have worked in his favor as well.

However, the curve ball was that this ornate floor was coated in an oil-based polyurethane approximately two years ago.

While there’s no definitive scientific answer to this yet, the general consensus seems to be that if you screen a floor with old oil-based polyurethane and coat it 36- 48 hours after the fact, you will have adhesion issues with the new coat of finish. In other words, when recoating oil-based polyurethane it is advised to immediately put down the finish after screening.

For some unknown reason oil-based polyurethane will “self heal” if the floor is left open after approximately 24 hours or more. The self healing can be describe as the profile scratch filling in and not having the needed profile for adhesion of the next coat. It’s also important to note that that this scenario only appears during recoat jobs. During brand new installs the oil-based polyurethane — even if it’s been cured for a week — is not the same as oil-based finish that is older, chemically speaking. At the time of this writing the consensus is that oil-based polyurethane that has been cured on the floor for over three months will present adhesion problems during recoats if you do your screening one day and come back to coat the floor a couple of days later.

If you do ever find yourself recoating oil-based polyurethane and you cannot apply the finish the same day as you screened, it is advised to lightly abrade the floor again if the floor has been left “open” for more than 18 hours.

This is a strange and odd phenomenon that can trip up even the most expert contractors and floor installation professionals. Hopefully sometime in the near future we will have definitive answers as to why this problem crops up.

Until then, feel free to call City Floor Supply at 800-737-1786 if you have any questions relating to the “self healing” problem with oil-based polyurethane finishes.