Many of the old Italian mandolins were polished with shellac, though later instruments, from perhaps the 1920s were also varnished. To restore a beautiful Italian bowlback to it's former glory, I feel it only right to French polish it. Thus this section is about French polishing... and yes, it has taken me some time to get it all together, as there is both a lot of mystique, a lot of mis-information, and for me anyway, a lot of new knowledge involved, though the actual process itself is relatively straight forward.
Varnish: is made by cooking one or more oils with certain natural or synthetic resins, which combine to then become a new substance. There 2 types of varnish;
| What is Shellac? Shellac is an
'evaporative' finish, which leaves a resin coating once the alcohol solvent
has evaporated. It is a natural resin secreted by insects called Lac
Beetles in northern India. They congregate in the tres in vast numbers,
and the resin is scraped from the branches, melted, strained to remove
impurities, and then solidified into thin sheets. These are then broken
to to form shellac flakes. It is said to take 1.5 million shellac
beetles to produce 1 ib of shellac.
How do I buy shellac? Grades include seedlac; buttonlac; *1 orange; Kusmi blonde; and bleached white.It is sold in flake or liquid form. For luthiers, the problems are compounded by the fact that, shellac has varied commercial applications. For instrument finishes, you are interested in liquid or dry flake shellac.
What is the shelf-life of shellac?
Are there different types of shellac flake? Yes. Shellac flakes come in a variety of forms, though each manufacturer seems to have their own names for each?!
Why de-wax shellac? Natural shellac comes with about 5% wax. This will lower its resistance to water; reduce its transparency; and prevent good bonding with non-evaporative finishes such as varnish. It can be removed after application, (see French polishing sequence) or before (by letting the solution stand for a few days, and siphoning off the solution, as the wax settles to the bottom.
Can I de-wax myself if I buy un-bleached shellac? Yes. Either by siphoning off the solution, leaving the wax at the bottom, or by repeated filtering with a fine muslin or cheesecloth.
How do I know if my jar of shellac is still ok? To test if it is still good, place a drop of shellac on to a glass surface and wait. Afetr aboiut 15 minutes, it should be dry to the touch.Leave it overnight. The following day scratch the surface, if it is still gummy, then it has gone off and should be thrown out.
How do I apply shellac? Shellac can be applied by brush, spraying or rubbing once it is dissolved in alcohol. Brushing and spraying can apply shellac quite thickly, but may need sanding in between coats. Rubbing (the process of French polishing) applies it very thinly, it doesn't need sanding between coats, but takes a long time to build up thickness. Rubbing can also be used as a method of 'finishing' shellac applied with spray or brush.
What does '3 lb cut' mean? 'Cut' is a term used to measure the strength of a mix of shellac; specifically, how many lbs of shellac are dissolved in 1 gallon of alcohol. The higher the number, the thicker the shellac.
Advantages of Shellac?
Disadvantages of Shellac?
|1. Preparing for French Polishing||2. French Polishing the process|
Problems with Shellac... (to be finished.....)
|What can go wrong? A variety of problems are
possible, but these can be minimised by the following;
most of the problems can be easily remedied by sanding out and starting again, but I will list some of the more common below and their causes.
|Burning: the finish is sanded through to the surface of the wood, or the pad passes over unset shellac and pulls it off.|
|Corners: difficult to rub in tight corners.|
|Holes: pinhole appear when the shellac is dry.|
|Shellac does not build up:|