Finishing and Maintaining Wood Floors
Part 2 – Surface Preparation

Introduction

Care must be exercised in preparing a wood floor since finishing will accentuate any defects, irregularities or roughness.  Even irregularities that can scarcely be seen before finishing become conspicuous afterward.  Nothing can be done later which will make up for the defects of a poor sanding job.  If much time passes between the final sanding and the application of the finish, some roughness may develop from raising of the wood grain (especially with hardwoods) because of changing moisture content.  Floors should be sanded shortly before finishing is started.  Floor sanding can be done by hand, but electrically driven sanding machines are used almost exclusively today.

Sanding Machines

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 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 sander 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 edges of the floor.

Sandpaper

Sandpaper acts by gouging fine slivers from the wood surface, leaving scratches, the size of which is governed by the grit of the sandpaper.  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 board.  Fine grits act slowly, but the scratches left are too small to see.  Scratches must be especially fine to escape detection on wood with close texture, such as maple, and must 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 grit are then removed by successive sanding with a finer grit.  The scratches left by the last paper should be too small to be observed even after the finish has been applied.

Sandpaper despite its name, is not made with sand.  It is made from abrasive grits, such as flint, garnet, or emery, which are natural minerals or from manufactured product such as aluminum oxide or silicon carbide.  The backing may be paper, cloth, or a combination of paper and cloth.  Flint is the cheapest sanding material, but it wears out most rapidly and is seldom recommended for sanding machines.  Garnet and aluminum oxide are most widely used for woodworking.  Aluminum oxide is generally preferred for high-speed machines such as floor sanders.

Sanding Procedures

Before beginning the sanding procedure carefully sweet all dirt, dust and other debris from the floor.  If nails are present, they need to be countersunk so the nail head is below the wood surface and will not damage the sanding machine.  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.  Of course, more passes with finer paper will result in a smoother finish.

Note:  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 red oak floors unprotected by finish because minute particles of steel left in the wood may later cause iron stains under certain conditions. 

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.