Preparing the guitar for polishing
After the guitar has been constructed it must be examined and sanded to remove all imperfections. Scrapers are better than sandpaper as they do not leave the wood pors clogged with dust. The dust hides the grain and beauty to some extent. Some polishers do not sand to very fine levels of grit, and prefer to only go to about 100p. I find this far too coarse and prefer to go to about 320, especially on the top of the guitar. When you are satisfied with the sanded finish, it is time to apply a thin coat of oil, either linseed or tung oil. I prefer linseed oil (boiled) and I thin it down with mineral turpentine and apply generously to the body of the guitar, except for the top. The oil will darken the wood and also highlight any imperfections in the preparation. Wipe off all the excess oil and remove any missed scratches or marks and then apply more oil, again wiping and leaving overnight to dry when happy with the finished product.
Now the shellac
The
first coat is very thin and is just a sealer that will also magnify and
highlight any imperfections. Thin the standard 2lb cut by about 3 to
1 for the sealer and apply to the guitar, doing the top first and then
the bindings. If you do not do it this way, the pad will absord colour
from the wood resin on the back and sides and make a big mess of the
top, especially if it is spruce. Allow this thin sealer coat to dry for
about 30 minutes. It is now time to fill the grain of the back and sides
and neck. The top will not require any grain filling.
Filling the grain
It is possible to fill the grain using already prepared grain fillers, but I prefer to use pumice, the traditional method. Pumice comes in about 4 grades and I use a number 2 pumice. Sprinkle some pumice on a piece of paper and dab the pad onto the pumice. Onto the pad drop some alcohol and rub this pumice/alcohol with your finger. This wets and "clears" the pumice. If you sprinkle the pumice directly on the work and try to use it that way, it will clump and not become clear and be visible in the final work. At this stage we do not add any shellac to the pad, just alcohol. Rub the pad in circular motions on the back and sides. This has the effect of sanding the wood and pressing the wood/pumice mixture into the pores. Add more pumice and alcohol as required. It takes about 30 minutes to get a retatively smooth surface on the back and sides at this point. I repeat, do not add shellace to the pad, only alcohol. Shellac will slow the process at this stage. Set the guitar aside and leave for a couple of hours or overnight.
The next day, scuff sand the body lightly and prepare for more pumice work. We have not touched the top yet and will not until we are happy with the grain filling of the back, sides and neck.
The problems with oil as a lubricant.
Some traditional French Polishing has been done with the aid of some typee of oil on the surface of the pad to reduce grabbing. This however can be very problematical with delicate items like guitars. French Polishing requires judicious use of the oil. Great pressure is needed to ensure the oil is displaced and the next layer of shellac adheres to the layer below. Novices who attempt French Polishing also often find themselves adding too much oil far too often. This results in the oil being trapped between the layers of shellac, causing problems such as soft polish and possible delamination of the layers. To avoid this I polish without any oils at all. It is a little more difficult to achieve a good finish off the pad, but it solves a lot of problems and I believe it is easier for the novice to learn because it eliminates the difficult task of "clearing" properly.
Clearing is one of the vital processes in French Polishing. It is the removal of the oil (that has been used as a lubricant) with alcohol. It is a difficult task to master because there is a risk of removing the polish at the same time. Without any oil there is also more friction which aids in the production of the high gloss. As with all of these types of techniques each person will develop their own methods. It does not mean that one is correct and the other wrong, it is just a matter of what works for one person will not necessarily work for another.
Practice is the main factor in this whole process. The beauty of French Polishing is that if you make a mistake it is fully and easily reversible. When you feel you have achieved a good flat surface, which may take two or three sessions with the pumice, it is time to start the bodying up stage. Put the guitar away to dry overnight and prepare for the next day's polishing.
