In Part 1 of this mystery we went over a request for assistance from a wood flooring consumer: Tammy was concerned she had a mold problem with her hardwood floors. Upon further inspection of the photos, if you’ve sanded much wood flooring, or perhaps simply looked into the bottom of a box of sandpaper, it’s likely you solved our mystery. The bulk of what appears to be mold on that wood floor is … black silicon carbide abrasive grit, like this:
Put another way, know what you get when you mix silicon carbide abrasive, popcorn ceiling nuggets, paint chips, shredded wood fiber and water-based floor finish? The “mold” in Tammy closet, that’s what! Take another look at the photos.
The cause? Ever run a drum/belt sander and heard “Tick. Tick. TICKTICKTICKTICK?!” Well, then you likely know what comes next ... “BOOM!” It is one of the scariest moments in floor sanding when a belt or sheet explodes. Then what happens? Abrasive is blasted in all directions; the stuff goes everywhere. A massive mess that, if not cleaned immediately, can get into your finish, and it’s possible coarse grits can be dragged into finer grits, which is a nightmare all its own.
Whether the mold-like patches were formed from flying abrasive debris, simply not vacuuming the closet properly, or both, the debris left behind ended up in the floor finish and accumulated in the corners of the closet. There, it became permanently encapsulated in dried/cured floor finish: 
The clues in her initial comment were quite revealing. Let’s take a look:
“I've also become very paranoid about mold because I have immune system issues.” That’s reasonable. Tammy couldn’t risk getting too close to the patches with her medical condition, and she had no familiarity with how this might relate to floor refinishing. And, from a distance, the patches did look exactly like mold!
“Keep in mind that we've been in our home for over 8 years … I do believe the floors were refinished before we moved in … these spots were on two totally different ends of the closet … It's pretty dark in there as well … there is no damage on the walls.” So, the floors were finished before the purchase of the home, and the dirty corners had been there all along. It is highly likely that if it were mold, it would have spread onto the base trim and other areas. Why was the person sanding so sloppy in the closet? Ever opened a closet to start sanding and nothing was ever moved out? I’ve seen floors finished without removing coats, shirts, and dresses. I’ve seen closets skipped completely! My wife is a Realtor. The DIY and hack handyman work that she has had me inspect ranges from unreal to outrageous. What people will do to make a home look “good” for a showing or home inspection can be unbelievable.
“When you feel the area, it feels raised, like dirt flecks. But I can't wipe them off … My husband took a good look and said he felt like it was sealed over … it doesn't come off … I feel like I would've noticed this before, but it's also on the sides of the closet, furthest away from the light.” The “mold” could not be “wiped off”. Her husband was able to feel the areas and take more detailed photos. He reported the patches felt hard and dry or “sealed over.” Tammy shares that she had never really looked closely into the closet corners (likely, not once in the eight years since the floors were finished). When I called, I asked Tammy if the patches felt like sandpaper and she told me, “Yes, that’s what my husband said.”
THAT is how we get it done here—wood floor pros working together, to bring it together. (If you’re interested in black spots that are more complicated, check out the article that started this entire exchange: “What Causes Mysterious Black Spots in Wood Floors?”)
Speaking of wood floor pros, check out our new podcast “All Things Wood Floor” hosted by yours truly (available anywhere you listen to podcasts, or below)! Let’s talk up some good flooring mysteries, share old floor stories, and see what we can learn together.
