Applying the finish can sometimes take you as long as constructing the project itself. Take your time and be patient. The first question to ask is: “What kind of finish should be applied?” There are many choices and many schools of thought on the subject.
There are film-forming and non-film forming finishes. Film-forming finishes are like lacquer, shellac, polyurethane and varnish. Non-film-forming finishes are oil and wax. Film-forming dries hard on the surface of the wood and gives varying degrees of protection against moisture, sunshine and chemicals. Non-film forming finishes soak into the wood and give much less protection against the elements and must be reapplied at regular intervals.
Non-film-forming finishes are easy to apply by just wiping them on and wiping off the excess. Film-forming finishes vary in difficulty to apply. Most, except the fast drying lacquers, can be applied with a brush. Fast-drying lacquers can only be applied with a spray gun and only in a carefully controlled environment.
The first rule of a good finish is to read the instructions on the can. It is vitally important to read the safety precautions. It is also a good idea to get the M.S.D.S. (material safety data sheet) from the supplier. It will identify the toxic elements in the product you are using and tell you what precautions must be taken. For most film- forming finishes this means wearing a mask that can filter out organic vapors.
When choosing a finish, look at the drying time. The longer it takes to dry – and some oil finishes will never dry – the more chance you have of foreign matter, such as dust, being deposited onto the surface. It is wise to work in the most dust-free environment you can find. Washing the floor will help to keep the dust down. Drying times are most affected by humidity. Applying a finish on a very humid day could lengthen the time considerably. In some cases, like when applying shellac, the humidity can make the finish turn cloudy.
Most film-forming finishes should be sanded lightly between coats. This is done to smooth any fibres which have raised up after the finish is flowed on. In the case of solvent-based polyurethane, it also helps with mechanical bonding between coats.
The water-based finishes raise the grain a great deal and must be sanded between coats and, perhaps, even rubbed down with 600-grit sandpaper after the final coat has cured. To lessen the grain raising you can moisten the surface of the wood by wiping with a damp cloth. This will raise the grain, which you then sand smooth on the final sanding just prior to the first coat of finish. Don’t sand too much on this final pass or you will expose new fibres to the surface.
Horizontal surfaces are the easiest to finish. Even if the finish is applied too thickly it will not run or drip. Intersecting surfaces, such as the inside of units where the top and bottoms meet, are the hardest due to double coverage at the intersection. To alleviate this problem you can finish the unit in pieces or in stages, so that you can work using as many horizontal surfaces as possible. For example, the unit can be dry-assembled using no glue, then taken apart for the finishing. Mask off the areas that will need to be glued, or scrape the finish off before final assembly. This has the added advantage of easy clean-up of excess glue during assembly. Any squeeze-out can be wiped up, without it soaking in, as it does on raw wood.
There are volumes of books written on the techniques of finishing, but the key element is patience.