The many different shellac polishes are all based on shellac (resina laccae), a deposit on the branches of certain trees, i.e. fig-, hibiscus- and ficus species, which are domestic in Thailand, India, Sumatra and other countries.
It is the dried secretion of different types of parasitic lac bugs (carteria lacca, coccus lacca). After their fertilisation the tiny insects attack the trees and wound the branches in huge quan-tity (to produce 1 kg shellac about 300.000 bugs are necessary). By sucking they cause the plant to ooze a resinous sap which they eat. By digestion it receives a chemical change and is secreted again. With this excrement the bugs construct cells similar in function to honey-combs in which they lay their eggs. The resin coats the branches totally by the time and solidifies gradually to crusts, whereby most of the insects die. The larvaes eat the honey like nutrient with which the cells were filled, finally break out of them and leave to renew the cycles.
Suitable trees are cultivated in plantations and are harvested twice a year, e.g. their branches are stroked with sticks in order to loose the crusts which are picked up. The so called sticklac (lacca in baculis, lacca in ramulis) is then crushed, sieved and repeatedly washed to remove impurities like dirt or parts of the plants and insects. The resulting product is known as seedlac (lacca in granis). Further refining by heat treatment, solvent extraction, filtration, bleaching by means of chemical additives and drying leads to the actual shellac.
Depending upon the kind of insects, trees, and refining methods it is available in different grades with different chemical composition (resin, waxes, oils, colour, etc.). To use it as a polish it dissolved in denatured alcohol (ethanol, spiritus).

There exist several techniques to coat a wooden surface with shellac polish. Like other lacquers or varnishes it can be brushed or sprayed. The traditional and most suitable one in guitar construction is the so called French polishing. This means that shellac is applied in low concentration and quantity using a rubbing pad and specific rubbing motions. The pad is made up of wool or wadding wrapped with a piece of cotton or linen. The process is time and labour intensive and very delicate. Interrupted by long drying times the finish is build up by numerous thin layers of polish.

Shellac has several advantages compared with modern finishes which are mostly sprayed (nitro-cellulose, acrylic, polyester, etc.):

It can be applied very thin and evenly (under 0,1 millimetre thickness), and due to its components (waxes) it is more flexible. The coat can manage the changes in form and volume of wood (shrinking and swelling as a result of its hyroscopic characteristic) and also it strengthens and supports the oscillation of the top without hindering it.

A characteristic trait of traditional French polishing is the considerable pressure with which it has to be applied (depending on work step). Thus the shellac particles are compressed to a homogenous density which can not be achieved by brushing or spraying techniques (where particles adhere each other only lightly). This increases the bond and anquorage to the wood.

If the finish was damaged, i.e. by crack repairs, it often can be renewed or restored with little effort, since the existing lacquer substance is soluble again and new layers connect excellent to older ones.

Also its aesthetics, transparency, gloss, and natural impression exceeds the synthetic finishes. In addition as a natural product most grades are non-toxic (however this is not true with some chemical refined grades).

Unfortunately there are not only advantages. The resins and waxes which favour the flexibility cause a higher sensitivity to heat (melting point 60°-100°C depending upon grade) and acid (i.e. in the sweat of skin), and also a lower hardness and protection against scratches and abrasion.

Some advises arise from that concerning the handling of shellac polished guitars: If possible the finish should not come in contact with bare skin, except of the neck. The neck can be wiped off with a soft cloth after playing.
Long extensive heat, i.e. by exposure of the sun, radiators or even continuos body heat should be avoided (soft leather rags or cloths can be underlayed additionally to the clothes). (See also site: Care tips for guitars)

Even with great care it can be necessary to let work over the finish at intervals of several years to keep its beauty and wood protecting function.