Poly repelled from seams of sanded wood floor

Q: I just sanded some old floors that I installed about 20 years ago. I sanded them with 40 grit, 60 grit, 80 grit, then buffed it with 100 grit screen. Then I put golden oak stain from Minwax. After the second coat of Lenmar oil poly, I went to put third coat on and I noticed all through the house at the seams and the butt joints the poly looks dry and the middle of the board is shiny. It looks like I missed polying the seams.

What could have caused this? I have picture if you would like me to text them to you.

A: Sounds like a contaminant. Were these floors waxed by chance? Because it can get into gaps between the boards and the solvent in the polyurethane causes it to bleed out and repel the polyurethane.

Q: We moved into a 50 year old house. Before moving our furniture in, I sanded the floors, applied several coats of Minwax oil-based stain, and applied three coats of Minwax polyurethane, also oil-based. Five months later, the polyurethane appears to be separating from the wood at the cracks between the planks in high-traffic areas and in some places the separation creeps toward the middle of the planks. The floor boards do move somewhat, but aren’t particularly unstable.

Is it common for the movement of boards when being walk on to cause this separation of the finish? Should the finish bond strongly enough to the wood to tolerate this? Maybe some contaminate is to blame? Is there any way to fix the problem spots so the bare wood isn’t exposed?

A: If the finish is peeling in the center of the board, you probably didn’t buff/scratch the previous coating enough to gain a good bond.

To fix it you will have to buff the finish down to the finish film that isn’t peeling. This will be tricky to do without removing the stain too. If it is just a couple of boards, maybe you can tape them off and just do those boards over.

Follow-up Q: The peeling doesn’t begin at the center of the board, it begins between boards where there is slight movement when the boards shrink in the winter. Only on some of the boards it spread from the cracks into the center of the board. Are you saying it’s common to see this issue at the cracks but if it spreads to the center it’s indicative of poor bonding? How can I tell if it’s just a layer of the poly peeling up or if its actually exposing bare wood?

Sorry for the persisting questions.

A: I wouldn’t say it is common to crack and peel at the edges. This could happen if there is serious shrinkage. The finish will only stretch so much and then it will fracture. But it won’t peel off the board unless there is a bonding issue.