What causes air bubbles in the floor finish?

Q imported from our old site, Face Lift Floors: What causes air bubbles?

A: Fisheye bubbles. You can get those in oil modified finishes as well as polyurethane, and the reason is usually

too rapid drying. The finish does not have enough time to flow and level before it sets up. I think that for any finish, prevent any sort of draft or moving air until the finish has leveled out. In the case of water-based finishes, give it an hour and then ventilate the work area. It is interesting that you mention the finish you used seemed to vary from can to can. It is also quite possible there was something wrong with the finish itself. That is not unheard of. You could be a magician, and no matter which way you stroke it, you can’t make it bubble free. I have been there too. I will say that water-based urethanes have come a long way. The top of the line finishes are cross-linked (you add a crosslinker). Very hard when cured but have certain procedural standards that must be followed in the application.

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Q imported from our old site, Face Lift Floors: Our 50-year-old oak strip hardwood floors were recently sanded and refinished with urethane. Due to some minor items, we had the flooring company come back a second time to fill in some gaps and buff and put another coat on. After the second time, we now have very visible air bubbles in our urethane finish (they were not present the first time). I suspect they hurried to complete the job. What is the best way to remove the air bubbles?

A: In my opinion, the air bubbles that have appeared after the last coat of finish (I call them fish eye bubbles) are not necessary, and not likely the fault of the person applying the polyurethane. Generally, as each application of finish is applied, it tends to dry faster. So the first coat takes the longest, with each following coat drying a bit quicker. In this case, it did not have time to flow out level and allow any air bubbles to break before it set up. I would suggest letting the floor dry for at least a week, and trying again. I would not use anything more coarse than a 180 grit screen with the polisher. After a thorough cleaning (vacuum up all dust and wipe with a cloth dampened with water) apply another coat of finish, with all windows in the area closed and all sources of ventilation stopped. After at least 2 hours, some ventilation can be allowed to help dissipate vapors. If there are any windows allowing direct midday sunlight to hit the floor during application, it is a good idea to cover them with blinds to prevent direct involvement of sunlight which would speed up the setting of the finish. What we are trying to accomplish is to reduce the speed at which the finish initially sets up. After it has had time to level out, open up some windows to assist in drying.

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Q imported from our old site, Face Lift Floors: When there are air bubbles in the polyurethane finish on a newly refinished oak wood floor, can you spot sand some of them by hand and then touch up that spot with another coat of polyurethane?

A: Very tough, if not impossible to totally blend such a spot in when using oil modified polyurethane. You may try buffing the entire individual board of the boards with fish eyes and applying a thin coat of finish to those boards. Otherwise, you may have to recoat the entire room.

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Related Q: After refinishing our oak floor, 3 coats in three days, there were rough areas and some bubbles. The workers came back to “fix” it and now there are bubbles everywhere. They now plan to buff it and apply one coat of BOBA instead of the polyurethane they used, which the rep said has had problems. Will that be adequate?

A: I have no idea what BOBA is. I have used Poloplaz Primero for years and it is so smooth anyone could apply it. A short nap roller works well. They only need to make sure after buffing that the floors and area are clean and air vents are vacuumed out also. No cross drafts from open windows while the finish is setting up. Not all finishes are equal and some can be problematic in their performance.

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