This is basically what I've done so far:

  • I sanded the floor down to 120 grit with a drum sander (did a pretty good, time consuming job)
  • Put 1 coat of polyurethane down - these problem-spots soaked it up completely
  • Put a second coat of poly down and this is the result
    • these problems start/stop at the edges of the boards - it seems specific to the wood

Elsewhere the floor looks really good. I can certainly sand & put a third coat down, but the fact that the finish looks like it is either cracking or not adhering to the floor has me concerned.

What could the trouble be, and what options do I have?

Update: A friend who does contracting work suspects the wood for this floor is odd-pieces and left overs; definitely possible from what I know of previous owners. The current thought is that some pieces were pre-finished with something that didn't accept my poly.

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  • Is the polyurethane water based, or oil based?
    – Comintern
    Aug 21, 2014 at 23:09
  • It's oil based.
    – doub1ejack
    Aug 22, 2014 at 0:35
  • How long ago was the wood flooring installed?
    – wallyk
    Aug 22, 2014 at 0:41
  • The answer posted by @TDHofstetter is probably correct then (I wasn't aware water-based polyurethane would do the same thing as oil-based if you didn't let it fully dry). Temperature, humidity, and the depth that it soaks in can drastically change the curing time. Solution is still the same though - remove the crazed polyurethane, let it cure a couple more days, then sand and put another coat down.
    – Comintern
    Aug 22, 2014 at 0:42
  • Before putting down the first coat of poly how did you "prep" the floor? Vacuum? Water? Solvent?
    – treeNinja
    Aug 22, 2014 at 14:45

3 Answers 3


There is something...in those pieces of wood. (Detailed enough for you?)

Ok, so I would have initially said temperature until you mentioned it was only specific pieces, which leads me to think moisture, but even that would cross pieces...

My best suggestion is sand those spots down, clean them with a solvent dampened rag, lacquer thinner maybe (don't pour it on) and then apply a wood treatment to the spots. Then re-coat.

Edit: To answer your question in the comments

Yes, should have mentioned that, you will need to sand before completing the steps I outlined. If it isn't real wide spread I would just use scraper to get the big chunks off and then a fairly fine grit sandpaper and do it by hand. If there is a lot I would go with a palm sander or belt sander. You don't want anything that spins it will leave marks that will be visible in the final finish.

  • We have to sand back down to the wood, right? What's the easiest way to do that? Can I just scrape the poly off the trouble spots and sand down with a handheld orbital sander?
    – doub1ejack
    Aug 22, 2014 at 15:43
  • see my edit to the answer
    – James
    Aug 22, 2014 at 15:56

That looks to my eye like a water-based polyurethane, and it looks like the first coat may not have fully dried before you shot on the second coat, then the floor temperature changed fairly radically (probably warming up) before the second coat was fully cured. The uncured first coat would have expanded with the warmth and tried to ooze out under the second coat. This would be more likely to happen in the most-porous areas than the less-porous areas, and indeed it doesn't seem to have happened on the quartersawn (with "flake" figure) areas but more in the plain-sawn (with "cathedral" figure) areas.

I'd try resanding just the affected areas, letting them stand untouched for about 24-36 hours, then reshooting them with a third coat.

  • We used oil based for both coats. The temperature changed maybe 10 degrees between coats. The floor dried for three days between coats.
    – doub1ejack
    Aug 22, 2014 at 0:36
  • Wow, I'm surprised - I'd have SWORN... but no matter. You used the same poly for both coats? They were thinned the same? Did you by chance use a fast-evaporating thinner to get the second coat to shooting consistency? Stirred really well before thinning? I don't think I've ever seen oil-based poly blister like that on red oak. You're sure no contaminants (especially silicone!) got onto the surface between coats? Silicone's famous for that, I keep silicone out of my shop - maybe why I've never seen blistering. Aug 22, 2014 at 0:43
  • Curiously, I've never seen water-based polyurethane do that, only oil based. Granted, I rarely use water-based.
    – Comintern
    Aug 22, 2014 at 0:53
  • I shot some kitchen cabinets a couple years ago with WB poly - had one that did that when I got too impatient. I was VERY skeptical about WB when I started, but I'm really starting to like it now. Still gotta' wear the mask, though. Goes on a little more water-clear than OB poly does, no yellowing at all. Aug 22, 2014 at 2:09

Looks like the raised grain on wood that has been wet. If it's water based poly, maybe related? I wouldn't know because I used water-based once and hated the appearance so much I never used it again.

I'll also offer that mixing and matching new and old wood is not forgiving as far as staining and top-coating. I would never undertake that myself since I have only done a couple floors, it takes a lot of experience to anticipate how difference species of different age and moisture content will respond to different products.

Dumb question but are you sure it's solid wood and not an engineered product with a veneer?

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