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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 colour. 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 are usually thinned varnishes which, 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 can be used successfully by most anyone. Fast drying sealers are 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.

Application Precautions

Certain application precautions are necessary to appropriately finish wood floors

Dust and dirt are an important factor in causing a rough surface. When applying the first coat of finish, 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 motes 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.
Most finishes will not stick to wax, oil and other materials which may contaminate the surface. Be certain that the finish is applied only to bare, clean wood.
The temperature of the floor, room and finishing solution should be about 21 degree Celsius or somewhat warmer to assure that the finish flows on evenly and cures properly.
Most finishes cure faster in dry weather. Therefore low humidity conditions are also ideal.
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.
Provide adequate ventilation to carry off any fumes.
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.

Source: Michigan State University