Photos courtesy of TODD SCHUTTE | Bona
Just when you thought the late ‘80s and early ‘90s were well in the rear-view mirror, the fabled “black spots” that mysteriously can appear in red oak are finding their way back onto center stage, along with their partner in crime – the increasing trend of white or natural-finished floors (versus the past quarter-century trend of light brown to dark to darker brown and gray-tone stains).
Here’s the typical scenario: You’ve just installed a new red oak floor or are getting ready to sand and refinish existing red oak floors. There are no apparent problems with the installation or the condition of the existing floor for the refinish, so you go ahead and sand the floor, and there still are no apparent problems. You get a beautiful coat of white stain applied to the floor (or leave it natural), and now you and your customer can hardly wait to get the sealer and finish applied to see the full reveal. You apply the first coat of waterborne sealer (to keep it white), and that’s when it transforms before your eyes. Mysterious blackish spots appear throughout the floor on random boards and in random patterns.
Your first thought usually is that you or your helper must have sharpened the scraper over the floor (or hit a nail), and these are the resulting iron deposits from the errant shavings. But look closer. From where do you see the spots emanating from? If they’re our mysterious dark spots, they’ll be board-specific and all originate in the soft grain vessels, elongating in the same direction as the grain (though some can spread a bit sideways as if smeared). So, what are these spots, and can we prevent them from showing up?
What are They?
The scenarios and chemical reactions that make up these spots have been around for as long as red oak. While there are various theories as to their exact origin, the general consensus is that the dark, gum-like substance that is the basis for these spots is formed by the tree’s defensive reaction to damage and/or the colonization of one or more microorganisms in that particular tree.
This substance forms and is stored in the open vessels of the soft grain to help prevent the spread of any fungal, yeast, or bacterial attack resulting from any damage or colonization. During kiln-drying, it does stabilize and becomes somewhat brittle, but the temperature used to kiln-dry wood flooring (160-180°F) is not high enough to make the substance insoluble (>212°F); thus, when alkaline and polar solvents (common in many sealers and finishes) are applied to the wood’s surface, the substance has a tendency to bleed out from the vessels.
Is There a Way to Prevent Them?
Photos courtesy of Bona
Since the “spot” substance originates in a “standing tree,” it can’t be remedied in the tree itself. Given that, and the fact that flooring isn’t kiln-dried to a temperature to eliminate the problem, you must discuss the possibility of these spots appearing with the floor owner when talking about their floor project from Day One.
The biggest challenge is the spots are only readily visible when the flooring is brand new (photo 1). If you’re good at spotting affected boards, you could separate them out during installation. Just don’t miss any because you won’t know until sealer/finish is applied when they’ll pop back out of the grain (which, of course, is too late). If an existing floor is stained a wood tone, it will be hard to pick out affected boards as the spots will look more like character marks than the dark spots taking away from your craftsmanship efforts. In addition, unlike the test photos, the affected boards likely will be few and far between and spread throughout the floor, making it very difficult and time-consuming to identify them all before sanding. Even if you identify and replace apparent black spot boards, other affected boards may become present in the floor (from the same tree or area) but just not showing their spots until you open up other affected vessels when sanding the floor. It can seem like you’re chasing your tail on this one.
As you can see from the test photos, once the floor is sanded (photo 2), the spots are difficult, if not impossible, to see because the dark smudges have been sanded off. The application of white stain (photo 3) didn’t pull them back out but just made the floor look fabulous. However, the black spots returned to the surface within 10 seconds of applying our first coat of waterborne sealer (photo 4). Two additional coats of waterborne finish didn’t make them worse, as the sealer did its job of sealing the floor against further penetration. Still, the final look probably is not what the customer was “expecting” (photo 5). Replace those boards? Sand the floor again? Spots appeared on different boards? Chase your tail?
While water-popping did make some of the spots appear (photo 6), it’s not enough that they would make the alarm go off in my head when inspecting the floor before staining. Plus, it still keeps me chasing my tail with pull and replace, sand, other spots appear, round and round we go.
So, for the moment, the best thing you can do is to discuss the possibilities of these spots showing up with your customer during the bid process, as it’s much harder to explain these away after the fact. However, through my testing, I think there are a couple of “systems” you could use to prevent or minimize these dark spots. I will cover this in my next article, which will be available on hardwoodfloorsmag.com.
Todd Schutte is the director of global eLearning for Bona AB in Englewood, Colorado. He may be reached at todd.schutte@bona.com.
Sources:
Black Spots in Red Oak – 1991 Gerald Thompson (Bona US) and Lidia Brito (Colorado State University)
Dark Deposits in the Grain of Red Oak – 1990 Gene Wengert & Fred Lamb (Virginia Tech)
