The Fundamentals of Sanding Wood Floors 2

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Whenever you think wood floors, think fine woodworking: building, sanding, and finishing wood furniture…only we do it on a much bigger scale. Everything we do with machines and scrapers on the floors originates from the fundamentals of using a handheld belt-sander, a planer, a chisel, and performing tasks like working to a line.

Our trade is not that much different from any other fine trade that works with wood. When we install new floors, yes they are semi-smooth and milled to NWFA standards, but we still need to sand them flat. We all have a different idea of what flat means based on our experience and education – those two elements affect how we look at the floor and what we can actually see. One of my guys said I had eagle eyes; I don’t. I am just trained to look for the imperfections. The most common floor to install, sand, and finish is still a solid oak floor. The big machine is the first one to touch the floor and is either a belt sander or a drum sander. Given the fact that most people use belt sanders, that will be what I will refer to in this blog.

A belt sander is just a spinning drum with a belt on it. The belt is a coated abrasive, for now, let’s just say for having a visual reference that there are small rocks on the belt. Some of the most common ones (the rocks or minerals) are ceramic, aluminum oxide, zirconia, and silicon carbide. I guess I will have to write some articles about that too, but not now.

Always vacuum the floor before you start sanding. The big machine should be run from the left side of the room to the right side. This is because of how the machine is designed. The right wheel of the big machine is directly behind the belt/drum, and the left one is not. When you cut a floor moving from left to right, the wheel that is not behind the drum always ends up on a flat surface. If you run it from right to left, the left wheel (which is not behind the belt) will follow the highs and lows dictated by the flatness of the floor, which can result in an uneven sanded flooring surface. It is much easier to understand if you flip your machine over and look at the wheel positioning.

Which grit should you use for cutting a new oak floor? Let me ask you a few questions before I answer:

  • Did you take care of the subfloor per NWFA guidelines before installing the floor?
  • Was your subfloor flat and with minimum deflection?
  • Did you buy your floor from a decent mill or did you buy the cheapest red oak selection your vendor carries?
  • Do you have quality belts for your sander?

I will answer those for you: you did take care of highs and lows, squeaks, and other issues with the subfloor before you installed the wood floor. You bought a high-quality milled floor. You have quality belts.

Fifty or 60 grit is your million dollar answer. FUNDAMENTAL: if you put a rough scratch in the floor you will need to sand it out with a finer grit later. Why put a 36 grit scratch on a brand-new clean floor? You don’t need to remove more surface material than necessary!

I am not here to argue with old school installers who still do 36 – 80 grit sequence. I have a lot of respect for those who have been doing this since before I was born (that is Aug. 26, 1976, by the way), but my respect is conditional on their acceptance of learning newer techniques of sanding floors. I’ve seen hundreds of contractors in training, at NWFA conventions, posting on social media, and more who are still doing things the way they were taught decades ago. Well, in Bob Dylan’s words, “the times they are a changing.”

Folks, the machines are not the same, the sand paper is not the same, the finishes are not the same, and finally, the EXPECTATION LEVELS are not the same. Go back to your first pass with the big machine at an angle with a 50 grit quality belt. Move from left to right while gently lowering and lifting the drum as needed. Imagine an airplane taking off and landing – that is the way the drum should move.

Safety note: Pay attention that your 220V cord is not around your neck while you are running the machine.

When you are done cutting the floor at an angle, the floor must be flat. That is, all the end joints and seams are flat, not almost or “I will get it perfect when I go straight.” FUNDAMENTAL: every step in the sanding sequence must make the following one easier, not harder. For example, if your end joints are not flat now, when your machine goes straight over them it will ride over the end joints oh ever so gently, just enough to create the perfect scenario for a dish out. Dish out that you will only see after the second coat of finish.

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