Floor sanding can be done by hand, but electrically driven sanding machines are used almost exclusively today. These machines are usually available from rental agencies and some paint stores/home centers which also supply the sandpaper. Some handwork is ususlly necessary in less accessible places.
Sanding machines may be either the drum type or disk type (floor polisher). In drum sanders the sandpaper is mounted on a cylindrical drum that rotates on an axis parallel to the plane of the floor. Thus, the sandpaper makes its scratches in straight lines in the direction of movement of the machine.
In disk sanders the sandpaper is mounted on a disk that rotates in a circle in the plane of the floor. As a disk sander is moved over the floor, the grits make spiral scratches that necessarily cross the grain of the wood. A drum sander, however, cannot reach the last few inches of floor nearest the baseboard. Electric edgers, which are small disk sanders, are available for sanding these edges of the floor, or they may be done by hand.
Sandpaper acts by gouging fine slivers from the wood surface, leaving scratches, the size of which is governed by the size of the grits on the paper. Coarse grits act rapidly, but the scratches they leave are conspicuous, especially if they cross the grain of the wood. Fine grits act slowly, but the scratches left are too small to see. Scratches are least noticeable when they run with the grain of a wood. Scratches must be especially fine to escape detection on a wood with close texture, such as maple, and must be still finer to remain unnoticed if they cross the grain of the wood.
In sanding a floor, time is saved by starting with coarse sandpaper to remove the grosser roughness and imperfections and to make the floor level as quickly as possible. The scratches left by the coarse grits are then removed by successive sanding with a finer sandpaper. The scratches left by the last paper should be too small to be observed even after the finish has been applied.
Before beginning the sanding procedure, carefully sweep all dirt, dust and other debris from the floor. "Set" all nails that may be protruding either in the floor or baseboard so that the sanding machine will not be damaged. Sometimes, only two sanding cuts are needed on a new hardwood floor, but if the floor is at all uneven or if a particularly smooth finish is desired, three cuts will be necessary. The first cuts should be done with a coarse or medium abrasive, always ending with a fine abrasive. A smoother finish will result if the final sanding is done with the floor polisher or disk sander. Of course, more passes with finer paper will result in a smoother finish.
After the second or third pass, the floor may be buffed with steel wool using a machine. However, steel wool should not be used on oak floors unprotected by finish because minute particles of steel left in the wood may later cause iron stains under certain conditions. When sanding strip, plank, or other flooring where all pieces run parallel to each other, all cuts may be made in the direction of the strips.
However, if the floor is at all uneven, one of the first cuts using coarse or medium paper should be at a 45 degree angle to the direction of the strips. This positioning will remove any peaks or valleys caused by minute variation in thickness of the strips or in the subfloor. When sanding parquet, block, herringbone and similar flooring, it is necessary to cross the grain of many pieces with each pass. In these cases, begin sanding on a diagonal from one corner of the room to the other. The next cut is started from one remaining corner to the other, and the final cut is made at approximately 45 degrees to the first cut (from one wall to the opposite wall).
Extra care should be taken to see that each pass after the first is deep enough to remove all scratches left by the previous sanding. The last pass should be made with relatively fine sandpaper.
Note: Regardless of the type of floor being sanded, an edger should be used after each pass to finish any areas which were not previously sanded such as edges, corners and areas around radiators. These areas may also be hand sanded. Before the sanding is considered complete, the floor should be inspected carefully to see that all blemishes and visible scratches have been removed and that a smooth surface has been produced. Defects can be seen most readily if the floor is viewed against light at a low angle of incidence so that any ridges will cast shadows. Any defects left at this time will show much more prominently after finishing materials have been applied.
Removing Old Finish
If an old finish cannot be satisfactorily repaired, a complete sanding of the surface and then application of a new finish may be necessary. Most flooring is 3/4-inch thick so they can withstand sanding. In these cases, make certain that all nails are countersunk and that the floor is as clean as possible before sanding. Use an "open face" paper to remove the old finish. The heat and abrasion of the sanding operation may make the old finish gummy and will quickly clog normal sandpaper. Once new wood appears, regular sandpaper may be used.
The number of cuts required to restore an old floor is largely determined by the condition of the floor and the thickness of the finish being removed. If the floor is badly scarred or warped, use as many cuts as necessary to get a smooth, unblemished surface. Make the first one or two cuts at a 45 degree angle with medium grit paper, and then follow the instructions given for sanding a new floor. If the surface is in good shape and has no thick build-up of old finish and wax, one pass with the disk sander and extra-fine paper may be sufficient. Just be sure that you have removed all the old finish.
Old finishes may also be removed with a non-aqueous (no water) varnish remover, after which the floor should be sanded as for new flooring. If the floor is less than 3/4 inch thick or if it is made from hardwood plywood, care must be exercised to prevent sanding through to the less desirable wood beneath. The floor thickness can usually be determined by removing a floor heating register or the shoe mold and baseboard so that an edge of the floor is exposed. When refinishing these floors a chemical varnish remover may be useful. It will also help to use a floor polisher or disk sander rather than the drum sander. Do not remove more wood than absolutely necessary.
As you can see wood floor sanding has multiple purposes. Now it's time for the finish!
This article has been contributed in part by Michigan State University Extension.