I promised a novice floor guy in New Jersey to write this article. He’s been having quite a problem, (and I’m sure customer complaints) because he has been leaving bubbles in the last coat of polyurethane. He would like a quick fix, but I’m afraid both you and he may be a bit surprised by the length and detail in this article. I sure that you will be satisfied that I covered all the problems associated with this fine and durable finish.
Polyurethane’s official name is actually Oil Modified Polyurethane, or OMP or OMU. That’s because it is mainly made up of oil (safflower or Soya oil), some metallic dryers (cobalt and manganese) and lastly some polyurethane resin to give it hardness once it dries. It’s also referred to as urethane, or even trade names like Varathane. You can be sure you are buying an OMU when the directions on the can suggest clean up with mineral spirits. Water based finishes are also called urethanes in some cases, but I will not talk about these finishes here. They have a completely different application method. It may become confusing when water based finishes call themselves urethane. But the water base finishes always suggest a soap and water clean up. Whereas oil based polyurethane’s require mineral spirits, naphtha or paint thinner for clean up.
But lets start from the beginning. If you are sanding a floor for a clear finish (some call it a natural finish) you must sand the floor at least 3 times. Start out with 36-grit paper, then 60 then 100 grit. If the floor is to be stained you must skip less grits and go with 36, 60, 80, then 100 or 120. The edges must be sanded thrice at least, with the final edging done with a 1/2-sheet vibrator sander with 80-100-grit paper. And for stained floors you must screen the bare wood with 100 grit screens just before staining. I’m not going to give you a sanding lesson in this article that will be for another time. Suffice it to say if your floor is not properly sanded clean and fine, your finish will always be rough. Oh, that’s why I always use 36-grit sandpaper on old and new floors. On old floor I need to remove ALL the old finish with the first rough sanding, no exceptions. And on new floors I want to level the boards quickly and easily.
So now that you have a nicely sanded floor, lets start and say you are doing a clear finish with 3 coats of polyurethane, what is the best way to proceed. First, sweep the floor if it is really full of dust, as this may be too much for your vacuum. Vacuum with really good industrial machine. It would be worth it to rent one for the whole job. There will be large particles of sandpaper grit on the floor, and your household vacuum won’t do the job. Be sure to use the wand of the vacuum to get into all the corners. Keep in mind, it’s best to go over the floor twice, this will ensure the floor is dust free. By the way, you need to finish your sanding the day you do the finish. Never leave a fine sanded floor overnight, any dampness in the air may raise the grain, especially in the summer.