Two things an improperly sanded floor will produce: unsightly sanding marks and premature wear of the finish coatings. First, let me explain a little about different grits of sandpaper, then the different types of machines that are used and finish with a sanding scenario.
Sand paper for sanding hardwood floors comes in different grits: 12, 16, 20, 24 are called “open coat”. These grits are used for sanding off of old wax or varnish. Most floors don’t require this aggressive off paper. “Coarse” paper comes in 30, 36, and 40 grit, these grits are mainly used for flattening of poorly milled flooring or flooring that has experienced a lot of movement, also good for floors that have a lot of scratches or UV damage. “Medium” paper comes in 50 and 60 grit, this is what we typically start sanding most jobs with. “Fine” is 80 and 100. And finally “Extra Fine” is 120 and 150 grit. We finish in the fine to extra fine range depending on the job and the look that is desired.
We use different types of sanding machines on the floor which require different types of paper and they have different uses. The “big machine”, which is basically a big belt sander, is used to sand the field of the floor. It is a rough sanding machine. The “edger” sands the edges of the floor along the walls, places where the big machine can’t get to. A “multi-disc” sander, we use one called a Trio, is used for fine sanding of the floor. A small random orbital sander, such as the Festool RO 125 Rotex, is usually used around the edges to clean up any sander marks left by the edger. And finally a buffer is used for inter coat abrasion and for fine sanding for those that do not have a multi-disc sander.
When sanding you don’t want to skip more than one grit. If you start out with a coarse grit, which creates deeper scratches, and jump to a fine grit you will only sand off the peaks of the coarse grit and not effectively remove the deeper scratch marks, resulting in a rough surface which will cause premature finish wear as the finish sitting on the peaks of the wood is worn off sooner.
Let me give you a scenario: take a red oak floor about 20 years old, 3 kids raised on it, couple dogs and cats, water spills, average life happens and the floor hasn’t had anything done to it in that time frame. At this time typically most of the finish has worn off in the walk ways and dirt has begun to be ground into the soft grain of the wood.
We would start with a 40 grit, skip 50 grit and sand with a 60 grit. Skip 80 grit and finish sanding with a 100 grit. This process will give you a beautifully, clean, new looking floor with minimal waste and provide a good surface for finish that will maximize it’s life.
Taking the scenario above some guys will do what is known as the “Chicago Special” (being the popular method of sanding in Chicago hence the term). They sand with the big machine using a 36 grit and then jump to 80 grit then screen it and coat it. This method of sanding will leave unsightly sanding marks and leave a rough surface for finish. It produces premature finish wear as the floor has peaks and valleys so the finish gets worn off the peaks allowing dirt to be walked in.
Another method which is popular around here, not as bad but still noticeable is to do a 50-100 sand. Being that 50 grit is a medium paper the initial scratch left by it isn’t so deep, but jumping up to 100 grit doesn’t effectively get the 50 grit scratch out.
A finish job is only as good as the sand job. For a longer lasting finish, thus getting the most for your money, make sure your floors are being properly sanded by a knowledgeable craftsman who understands about the sanding sequence.
(information contained here comes from years of experience as well as the NWFA Sand and Finish Guidelines)