There are many different types of timber stains such as varnish or shellac and they are made for interior or exterior use. These finishes usually darken the timber but bring out the grain and they are available in a range of different colours and textures that also help to preserve the timber.
Finishes like these such as shellac or varnish are usually applied with a brush because it is the simpler and easier method for most people when applying this type of finish.
You also need to apply around two or three coats depending on how well the wood absorbs the finish or how dark you want your timber to be.
Before the finish can be applied the excess dirt and dust must be completely removed with a slightly damp cloth so that the finish is applied evenly and no dirt is trapped underneath the coat.
You must also sand the timber smooth to remove any scratches, lumps or creases so that an even coat of finish can be applied. Later when the finish has dried you can add a clear coat to further increase the timbers aesthetics or to help protect it better.
If you don’t have much experience working with a finish such as these I would suggest applying it with a brush because it is usually the simplest and most effective method especially for beginners.
If you do not wish to attempt applying the finish you can pay for it to be done professionally which often gives your timber a glass like appearance and smoothness.