Floor-ology / Wood Floors and Finishing

Wood Floors and Finishing

Floor sanding is the first step in refinishing hardwood floors.

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 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, can not 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 sandings 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.

Sanding Procedures

Before beginning the sanding procedure, the floor needs to be carefully prepared, ideally by a sand and finish professional.

What we’ll do is clean the floors then “set” all the 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 are typically 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, depending on the condition and wood species/thickness. It’s absolutely critical to work with a professional that has experience with various types of wood species!

For example, 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!

Sanding Strip, Plank & Other Similar Floor Types

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.

Sanding Parquet, Block, Herringbone & Other Similar Floors

When sanding parquet, block, herringbone and similar flooring, it is necessary to cross the grain of many pieces with each pass. In these cases, we 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.


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.

Old Finishes

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 it can withstand a number of sandings. An “open face” paper is used 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, many cuts may be necessary to get a smooth, unblemished surface. The first one or two cuts need to be made at a 45 degree angle with medium grit paper. 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 as long as the old finish is entirely removed.

Finishing a Wood Floor

Finishing a wood floor is perhaps one of the most critical but rewarding steps.

Finishes are applied to wood for two principal reasons:

First, a finish should protect the wood from damage such as stains, moisture and mechanical wear.

Second, a properly applied clear finish will accentuate woods’ natural beauty and color.

Penetrating seals (sealers) and surface finishes are the two principal types of protective coatings used on wood floors. Either will give satisfactory performance if applied correctly.

Penetrating Seals

Penetrating seals are probably the most common finish on residential floors. Sealers when applied to wood, will penetrate into the wood pores on the surface. The result is usually a low gloss or satin finish that wears only as the wood wears. The eventual effects of traffic are far less apparent than with other finishes that only coat the surface. Scratching and chipping of this finish is not a serious problem. One coat of a penetrating sealer can give satisfactory performance, but two coats are generally better.

There are two basic types of sealers. Normal (slow drying) sealers and fast drying sealers (the latter of the two is more difficult to use since it is easy to form lap marks or a splotchy appearance).

Therefore, they are usually applied only by experienced professionals.

Surface Finishes

Surface finishes which are relatively easy to apply and will give satisfactory service include polyurethanes, varnish, shellac, lacquer and others. The polyurethanes are some of the most popular surface finishes because of their high resistance to moisture, mechanical wear, stains and spills. They are available with a high gloss, or satin or matte finish. Polyurethanes are either oil modified or moisture-cured.

Varnishes can also give satisfactory performance. However, varnishes do have a greater tendency to scratch, and worn spots are difficult to patch without showing lines between the old and new finish. Varnishes specifically designated for floors tend to be more durable. A glossy or matte finish is available. Varnishes may be based on phenolic, alkyd, epoxy or polyurethane resins. Shellac and lacquer are sometimes used as floor finishes. These finishes will dry rapidly, and more than one coat can often be applied in the same day.

However, shellac and lacquer are not as resistant to moisture, spills and mechanical wear as are the penetrating sealers, polyurethanes and varnishes.

Surface finishes will usually give a longer life than penetrating sealers without any attention other than regular sweeping or dry mopping. However, when surface finishes must be renewed, it is usually necessary to refinish the entire room.

Staining Wood Floors

In most cases, it is preferable to maintain the natural color of hardwood floors by using a clear finish. However, if a color different than the natural wood color is desired or if the natural wood color is too variable, a stain may be used.

Stains do not penetrate wood deeply, and they may fade with continued exposure to bright light.

Open grained woods such as oak, ash, pecan and walnut will take stain easily while the close grained woods such as maple, and to a lesser extent, birch and beech, will take stain much more slowly.

Soft-woods do not stain well since the less dense springwood easily stains dark whereas the dense latewood will hardly stain at all. Be certain to use “non-grain-raising'” stains. Take the same care in cleaning and preparing a surface to be stained as would be done in finishing it.

Oil-based pigmented wiping stains are probably the most common. The pigment collects in the open pores of the wood and thus accentuates the grain pattern and alters the wood color. Pigmented stains are usually applied by brushing.

Colored or pigmented penetrating sealers are also available. In this case, the pigment is mixed with the sealer, and both are applied at the same time. Pigmented penetrating sealers will not obscure the natural wood grain or shorten the life of the floor. Varnish stains are similar to penetrating sealers since the coloring pigment is formulated with the varnish. Therefore, the wood is colored at the same time it is finished. Since the coloring pigment remains in the varnish as it cures on the surface, much of the natural wood grain and color is obscured.

Certain application precautions are necessary to appropriately finish wood floors, which is why this is not a DIY-recommended activity!

1. Dust and dirt are an important factor in causing a rough surface. When applying the first coat of finish, one must be certain that the wood is perfectly clean and free of dust, dirt and other foreign materials. Dust and dirt must also be removed from cracks or other floor irregularities. The walls, windows and doors should also be cleaned to keep dust mites from dropping into wet finishing materials to mar their appearance. A painter’s tack rag or turpentine-dampened rag will help pick up much of this dirt. A careful cleaning is also necessary before a second or third coat of finish is applied.

2. Most finishes will not stick to wax, oil and other materials which may contaminate the surface. The finish must be applied only to bare, clean wood.

3. The temperature of the floor, room and finishing solution should be about 70 degrees F or somewhat warmer to assure that the finish flows on evenly and cures properly.

4. Most finishes cure faster in dry weather. Therefore low humidity conditions are also ideal.

5. A rough finish can also result if dust or small piece of dried finish are transferred from an old applicator or from a partially used can of finish. For each job, it is probably best to start with a new applicator and supply of finish.

6. Provide adequate ventilation to carry off any fumes.

7. Application of finishing materials should begin promptly after sanding so that there will be no time for changing moisture conditions to raise the wood grain.

Penetrating Sealers

Penetrating sealers are best mopped on using a clean string mop or long-handled applicator with a lamb’s wool pad. Generous amounts of the sealer are applied, making sure that final stroking is in the direction of the wood grain (if possible).

After the first coat has dried it gets buffed with No. 2 steel wool. Buffing is done with an electric polisher equipped with a steel wool pad.


Polyurethanes may be applied using a brush or lambswool applicator. Because polyurethanes are a surface finish, care should be taken to work along the grain. Polyurethanes should be flowed on in a continuous manner so that the leading edge does not have time to dry out. After the first coat is thoroughly dry, it is buffed with steel wool, dusted well and then, usually two additional coats are applied buffing between coats.


Varnishes are usually applied with a brush and flowed on evenly and smoothly. The first coat can be thinned lightly so that it will penetrate into the wood like a sealer. After the first coat has dried, it is smoothed with fine sandpaper, dusted well and then a top coat is applied at full strength.

For the final touch of beauty and to protect the finish, one or more coats of good wax can be applied to the floors.

The wax is applied after the finish coat is thoroughly dry and then it is polished with a machine buffer. The wax will give a lustrous sheen to the floor and form a protective film that prevents dirt from penetrating the finish. Some manufacturers of urethane finishes do not recommend waxing, especially for commercial jobs, because wax may make the floor slippery.

Wood floors finished with penetrating seals are not too difficult to repair should they show early signs of wear in the traffic channels or become stained or water damaged in localized areas.

Floors finished with polyurethane or varnish can also be repaired, but lap marks or a splotchy appearance are more difficult to avoid. Floors finished with lacquer or shellac are nearly impossible to repair successfully.

Finishes are best renewed when they begin to show signs of wear in traffic channels but before the bare wood is exposed.

In this case, the floor must be cleaned of all dirt and debris, and all floor wax must be removed as it may interfere with the drying and adhesion of any new finish.

If a surface finish such as polyurethane or varnish is being used, it is applied to the worn areas first. After the first coat is thoroughly dry, a second coat is applied over the entire floor.


This article was written by Anne Field, Extension Specialist, Emeritus with references from Finishing & Maintaining Wood Floors North Central Regional bulletin, Purdue Extension and the U.S. Forest Service.