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I moved into my home in May 2013 and had the Oak Flooring sanded and re-treated as it was a bit stained from previous owners (the flat is only 5 years also thought so the floor isn't very old at all).

Since the summer, these black marks have started appearing all over it, the worst appear to have cracks/holes in them. At first I thought it might me the knots in the wood but having kept an eye on them coming its not as they are appearing in places where there are no knots in the wood.

I do have underfloor heating (water) and am starting to get concerned there may some moisture underneath causing these? What do you think?

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    Do you have a photo of where this isn't a knot in the wood?
    – BMitch
    Commented Mar 4, 2014 at 13:55
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    Black holes are formed when there is too much gravity. Contact your gravity supplier, and have it reduced.
    – Chris Cudmore
    Commented Mar 4, 2014 at 14:10
  • @ChrisCudmore Yes, but by the time you see them, you are already sucked in.
    – bib
    Commented Mar 4, 2014 at 14:26
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    Is it possible that the previous owner had the knots filled/stained to match the rest of the floor? The process of refinishing the floor has removed the cover-up, and exposed the underlying knots.
    – Tester101
    Commented Mar 4, 2014 at 16:43
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    I have a similar problem. The black holes are not knots as they have only appeared 4 years after I built a cabinet from oak. I did use wire wool when smoothing the wood down before applying polyurethane varnish. However, I find that the holes are only appearing on certain areas of the cabinet mainly the door panels. The oak for these panels were purchased separately from the oak I bought for the main cabinet. I am concerned that the holes may be caused by borers. I know this doesn't answer your query but I do realise that this is a problem which is not related to knots.
    – user21459
    Commented May 9, 2014 at 19:36

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The tiny black specks that are not knots in your pictures are most likely from small particles of steel or iron causing staining. I wonder if your refinish process used steel wool (not a good idea on oak unless you are trying to stain it black.) The larger features with actual holes are, of course, knots, so if you have "holes that are not knots" you have not shown any pictures of them.

That new picture is a bit hard to see detail in (as you know) but there's certainly something knot-loke going on on the grain around it, so it probably is as well. Sometimes on an older floor that's been sanded down a lot you start to see stains from the steel nails, but that seems less than likely here.

To skip right to your water concern - probably not, since there's no sign of the floor buckling or failing to lay flat - wet hardwood floors tend to look like #expletive omitted# in short order as the bottom swells and the boards cup.

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  • Have just added one that I didn't think was a know, (not brilliant admittedly) will get a more recent one when I get home
    – user20252
    Commented Mar 4, 2014 at 14:41
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My guess is that the knots were previously filled with a wood filler like below which came out when the floors were re-sanded. You could buy several colors of the filler, mix them if you need to better match the floor stain, apply by firmly pressing down with a putty knife, then wipe with a paper towel, and return any unused putty. enter image description here

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