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No Clear Answer on Black Spots on Red Oak Wood Floor

The Problem

I received a call from my sanding and finishing foreman about "black dots" all over a newly sealed red oak floor we had installed. I drove out to the job, and sure enough, little black dots appeared in certain areas of the flooring. They appeared to be mold or something closely related. At the time, I had been installing and finishing flooring for about 15 years, but I had never seen anything even remotely resembling this problem.

The Procedure

We installed 1,000 square feet of unfinished select red oak strip flooring in a new home. There was nothing out of the ordinary about the job; the installation was done with the subfloor, wood floor and home at proper moisture levels, and the sanding procedure was routine. Then, waterborne sealer was applied, and the black dots appeared overnight, affecting about 10 percent of the boards.

The Cause

I called my distributor and the finish manufacturer, both of whom seemed certain that the spots were due to metal filings, possibly from steel wool or scraper blades. Since no steel wool had been used on the job, I surmised that sharpening scrapers over the flooring must be the cause, even though this had never caused us problems in the past.

Weeks later, when we sanded and finished another floor with waterborne urethane, guess what happened? More patches of black dots appeared on our flooring. Once again, new, unfinished, select red oak flooring was involved, but this time, my crew had been careful not to sharpen their scrapers over the flooring. Since metal filings from scrapers could not be the cause, I decided to check for more unusual sources of contamination. We examined machines, finish and even brushes and applicators for anything that could be causing the problem. Again, we found nothing, and again, both the distributor and manufacturer assured me that we must be doing something to contaminate the flooring or the finish.

Having eliminated all the sources of contamination I could think of, I began asking others for their thoughts on the matter (and I continue to do so to this day). I would guess that I have asked about 25 wood flooring professionals about this problem, and only one person has ever offered a possible cause other than metal contamination— this person suggested that one possible explanation was a fungus in the stand of trees in the forest. Although it is not a proven fact, I think that this was the likely cause. At the time, however, all I knew with certainty was that metal contamination was not the cause of our problem, and that we still had a problem.

For me, a problem is defined as serious when it meets two conditions: The first is that lights must be strung at the job site because we are working into the night to resolve the problem. The second condition is that both my sanding and finishing foreman and I are personally working on the floor. By these standards, the black dots in the flooring were a serious problem for our company. Finding a definite cause would have been nice, but finding an immediate solution was paramount.

How to Fix the Floor

When the first case of mysterious black dots occurred, a customer was waiting to move into the home. To eliminate the dots, we tried scraping off the offending spots and re-applying finish. The next morning, spots were once again present, but not as many as before. We repeated the scraping process but decided to see if a different procedure would produce different results. This time, we tried applying a neutral, oil-based sealer to the scraped floor. In this case, this step turned out to be the key to prevention of the "black spot disease."

For awhile after that, we used a neutral, oilbased sealer on new red oak flooring before applying water-based urethane. We are not sure exactly when, but over time, this problem disappeared just as mysteriously as it appeared. The memory of hand-scraping black dots off of the flooring waned, and we returned to using waterbased sealers again with our water-based finishes. Hopefully, we have seen the last of these black dots.

In the Future

Sometimes, things happen that nobody, even the experts, can explain. The cause of the black dots on the flooring has never been pinpointed. Problems, causes and solutions usually happen in that logical order, but sometimes, a solution must be devised even when the cause is unknown. Such was the case with this mystery of the black dots.

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