All my instruments are hand finished. The finish exists to protect the wood from dirt and moisture as well as beautifying it. In the main, I use French polish (shellac), which is a totally organic material derived from the secretions of the larvae of the lac beetle “Coccus lacca” in India and other parts of south-east Asia. The material is gathered and processed in various ways, ending up with thin sheets that are crushed into small pieces. The crushed shellac is then dissolved in alcohol and applied with a ‘rubber’ ( a pad made from cloth wrapped around absorbent material) using a little mineral oil for lubrication. After filling the grain, when necessary, the polish is built up in many thin layers until the desired surface is achieved. I much prefer the softer look of a hand rubbed finish with its minor inconsistencies, to a mirror flat hard gloss finish. The polish doesn’t chip and can be repaired if it gets worn away. It is thin and flexible, so it doesn’t impede the vibration of the guitar body. Its only shortcoming is that it is relatively soft, so will take a fingernail mark easily. Some guitarists agonise over every mark, while others mark instruments everywhere with gusto and aren’t bothered at all. There is a happy medium somewhere between the two extremes.
Some people have a certain acidity level in their perspiration that eats through polish ( and oil varnish), which then allows moisture into the grain and blows out the grain filler. For those people I’d recommend using Danish oil for the back and sides, which seems to be a little more resistant and can easily be built up again.