What is French Polish and why use it?
The answers are simple. We use French Polish on high quality guitars simply because there is no equal available today, acoustically speaking. French Polish will last a very long time, but it can be damaged easily, especially when new. But if it is the best acoustically and the guitar is expensive then proper care is called for.
French Polishing is the art of applying the shellac in very thin layers to the wood using a pad (sometimes called a muneca or tampon). This process is described in the following article. The pad is usually a piece of cotton or linen cloth wrapped around cotton or woolen padding.
There are many tales around about what shellac is, and they vary widely. Shellac is a substance
that a small insect in Asia, the Coccus lacca, exudes after sucking the sap from tree branches. 
The
exudate is used to construct the shelters or coccoons on the branches of the trees. Humans
collect this substance from the tree branches. In its raw form this is called sticklac.
From here start the various purification processes. The purification can be simply a washing
to remove as much bark and insect remains as possible. In this form the shellac is now
called seedlac. The next step invloves either chemical or mechanical purification. Mechanical
purification is done by melting and then filtering through hessian cloths or sacks. While
still soft the molten shellac is stretched into thin sheets and allowed to cool.
Shellac comes in many grades and the more purified the shellac the clearer or paler it becomes. Chemical purification or bleaching is done by dissolving the shellac in alcohol and bubbling chlorine gas through it and then evaporating off the alcohol. The shellac also becomes softer as it is purified and bleached. Dewaxing can be carried out at this stage as well, but I find it easy to dewax the shellac as required.
The most common is called orange shellac. After the shellac is heated and filtered it is
poured into thin sheets and allowed to harden.
Then it is broken up into small flakes. The picture here shows orange shellac on the right
and a blonde dewaxed shellac on the left. In the centre is a jar with shellac on the bottom,
and alcohol on the top. The shellac dissolves in the alcohol to produce the liquid we use
to produce the French Polished finish. I generally fill the jar about 1/3 with the flakes
and then fill the jar with alcohol. This yields what is called a 2lb cut. In old terms this
meant that 2lbs of shellac was dissolved in 1 gallon of alcohol. I never need those quantities
at any one time, so my volumes and weights vary accordingly, but still yield the same ratio
of shellac to solvent. This mixture is our basic ingredient.
Alcohol usually contain varying amounts of water and the higher this contamination the less suitable it is for French Polishing. As the shellac is applied the solvent (alcohol) evaporates leaving behind a thin layer of the shellac. The water does not evaporate out with the alcohol and this results in a problem called "soft polish". Soft polish is exactly what it says it is. It marks easily and is generally unsuitable as a coating for instruments and is the most common complaint for those new to the art of French Polishing.
There are many other causes for soft polish, one of which is the use of too much oil during the polishing process. I will talk more about this problem later.
On the guitar shown in this article I am using the blond shellac because the guitar has a
spruce top. Orange shellac can look very ordinary and patchy on beautiful spruce tops.
I prefer the spruce to age to a golden colour naturally under blonde shellacs. Also if you
are not very experienced at French polishing the orange shellac can look uneven and unsightly
on Spruce. This photo shows the same bottle of shellac, but after the shellac has dissolved
some time later.
