French Polish Technique
French polishing is a wood finishing technique that results in a very high gloss surface, with a deep colour and chatoyancy. French polishing consists of applying many thin coats of shellac dissolved in denatured alcohol using a rubbing pad lubricated with one of a variety of oils. The rubbing pad is made of absorbent cotton or wool cloth wadding inside of a piece of fabric (usually soft cotton cloth).
French polish is a process, not a material. The main material is shellac, although there are several other shellac-based finishes, not all of which class as French polishing.
The finish is considered by many to be among the most beautiful ways to finish highly figured wood, but it is also recognised to be fragile. It is softer than modern varnishes and lacquers and is particularly sensitive to spills of water or alcohol, which may produce white cloudy marks, as does heat damage. However, it is also simpler to repair than a damaged varnish finish, as patch repairs to French polish may be easily blended into an existing finish.
French Polishing Process
The process is lengthy and very repetitive. There are also many similar variations in schedule and technique. What is described here is one such schedule. The finish is obtained through a specific combination of different rubbing motions (generally circles and figure-eights), waiting for considerable time, building up layers of polish and then spiriting off any streaks left in the surface.
The fad is first used to place a thinned coat of shellac. Once fully dry, thicker coats follow with small amounts of superfine pumice, a crushed volcanic glass. The pumice acts both as a fine abrasive and to fill the pores of open-grain woods, though pumice tends to be messy to work with and many modern woodworkers simply use fine-grained sandpapers to smooth off the earliest layers of shellac— the resulting shellac powder will then fill in the wood pores at least as well and often better than pumice, and the subsequent shellac coats will liquefy and surround this dust, filling in and sealing the wood pores as it does so.
The 'fad' is often lubricated with an oil that then becomes integrated into the overall finish. This helps to prevent the 'fad' from lifting previously applied layers of shellac. Typically, softer/ thinner oils such as mineral oil will produce a glossier though less durable finish whereas harder/ more viscous oils such as walnut oil and olive oil will produce a more durable finish. Although boiled linseed oil is commonly used as a wood treatment, it is too viscous to use for French polishing.
There are two main variations to French polishing with regard to the use of pumice/ abrasives.
In the original 'French Method', both shellac and abrasive are applied and worked together.
In the 'British Method', shellac and abrasive are worked alternately.