While selecting a clear finish, the qualities required from the finish need to be considered:
- It must protect the wood.
- It must be durable enough for the intended application.
- It should be as easy to apply as possible.
A finish that has maximum resistance to moisture vapor exchange, protects the wooden items for the long term. Thicker finishes tend to slow down this exchange more.
The wood finish needs to be durable to protect the wood beneath. Durability has more to do with the chemistry of the finish than the number of coats.
Ease of Application
The ease of application is necessary to achieve a professional finish. Slow drying finishes like polyurethane and varnish are relatively easy to apply with a brush. Faster drying finishes can be difficult or impossible to apply without a spray gun. Oil finishes arent affected by dust since they fully soak into the wood
Polyurethane it is a hard plastic coating. Polyurethane is a durable, fast-drying matte finish. It enhances a woods natural amber or yellowish color.
Linseed oil which gives a fantastic finish, was more in use in the earlier times. This oil is made from the seeds of the flax plant. The raw linseed oil, never dries properly and becomes gummy and sticky, hence it is advisable to use Boiled Linseed oil. Linseed oil without this additive can take over a week to dry.
Pure Tung oil is pressed from the nuts of the Tung tree. This finish does not require drying additives and cures in several days. A light scratch in the finish can be worked upon by applying another coat of the finish with a piece of cloth.
Varnish is the most commonly used protective finish and the level of protection increases with additional layers. Being slow drying, varnishes allow dust and dirt time to settle thus, it is best to finish the project in a clean dust-free room.
Varnishes are produced by cooking an oil and mixing it with a resin such as synthetic alkyds, phenolics, and polyurethanes.Achieving a perfect varnish finish is a combination of skill and experiences. Varnish is usually applied with a brush using long steady brush strokes and should be wet sanded between coats with a fine sandpaper.
Lacquer is flexible, durable finish which is very easy to keep up. Regular lacquer dries quickly so it is applied with spray equipment. The spray is very flammable, so one must be cautious. Lacquer is used most often in furniture factories because its fast drying properties reduce dust related finish problems.
Shellac is one of the few natural resins that provides a modest amount of protection. It is a durable and dark finish but it is very susceptible to water and alcohol(alcohol dissolves shellac). Being alcohol based, shellac dries very fast. It comes in two forms - flaked and liquid. Both should be mixed with alcohol to thin it out. Shellac is often too thick to apply with a brush and requires thinning with denatured alcohol before it can be brushed. It is available in a range of colors from clear to an orange/amber color.
Because of increasing environmental concerns a new class of finished has been developed. The water-based finishes are often marketed as polyurethane, varnish, or lacquer which is untrue since all of these finishes are solvent-based. Water-based finishes are basically latex paint without a pigment. They tend to hold visible brush marks and are less durable than the more conventional finished. They also tend to bubble with brushing.