ETUDE: Applying Finish
KEN: Ah, finishing! Maybe the most vexing, uncertain, puzzling and potentially dangerous of all the instrument builder’s chores.
The luthier’s “Sword of Damocles.”
For many centuries, finishes and paint coatings were all made by grinding up, pressing, boiling, fermenting or cooking up some combination of plants, animals, and minerals.
Broadly speaking, here are the old school recipes:
Varnish: processed pine tree sap, flax (linseed) oil, minerals. Note, “Lin” is French for Linen
Shellac: Lac bug secretions and carcasses, alcohol
Paints: oil and minerals/plants or eggs and minerals/plants
Gesso: Rabbit skin glue and chalk or marble dust
Get the idea? Just mix stuff from your kitchen with stuff you find in the woods and smear it on whatever you've built!
There came to be many styles and recipes for finishes, and specialists applied them to furniture for royalty as well as to musical instruments. For example, shellac was used by some classic period violin makers, the solids coming from lac bug secretions imported from what we now call India or Thailand beginning in the early 1600s.
This precious organic resin was thinned with alcohol and either padded or brushed, and became known in the instrument world as “spirit varnish.
Northern Italy is a great climate to grow flax, a prolific cash crop, which can become linen fibers for clothing or cordage, or pressed to express linseed oil. The place is covered with pine trees too, which are easily tapped for sap, then distilled into turpentine.
Turpentine, a versatile oil-based solvent, was always a key ingredient in varnishes, further processed to become beautiful colored resin, and later used medicinally (now we know better), to make camphor, and, much, much later, Vick’s VaPoRub.
For the local luthiers, these were cheap, homeboy ingredients, and were all that was needed to produce “terpene resin” varnish, the production of which involves boiling age-thickened pine sap to produce a kind of peanut brittle resin which is then ground up and mixed with linseed oil.
A significant safety problem in the making of this lovely finish is that the process of boiling the sap involves courting a very, very dangerous violent exothermic event, so natural selection may have had a hand in the ultimate quality and availability of this finish.
The boiling process must be done outdoors, and it makes deep frying a turkey look safe and easy.
If you can’t help yourself, here’s some excellent advice, but this is a big commitment, and pretty scary. I’ve done it, and I’m done with it. Sure is beautiful, though.
http://www.scavm.com/Fulton.htm
If you want to skip the potential for disfigurement, here’s an option, let Joe Robinson make some historic varnish for you!
http://violinvarnish.com/contact/
These recipes evolved and improved over the centuries and continued to be used on musical instruments until 1921, when nitrocellulose lacquer came into commercial use. Mandolin and guitar makers were early adopters.
Spray guns were developed in 1924 and quickly replaced brushing and padding.
Nitro, as it’s now known, had lots of advantages for production line builders, and some drawbacks, like yellowing and crazing, which were minimized by better living through chemistry.
I once read that nitrocellulose lacquer was invented by chemists charged with finding a way to make a peaceful, alternative use for a whole lot of “gun cotton,” a primer/explosive widely used in WW1 to initiate cannons and naval guns, which, after “The War to End All Wars,” would no longer be needed by humanity. In a recent search, I was unable to verify this, but it’s still a great story, isn’t it?
For forty years, nitro was used for autos, motorcycles, and anything else that needed a slick finish. It was gradually displaced by baked enamels, then acrylics, urethanes, polyesters, epoxies, ultraviolet curing agents, etc., etc.
These wonder finishes each have an Achille’s heel or two, and I am told by repairmen that nowadays the first problem when touching up is correctly identifying the type of finish used, not only to get the “look” correct, but to assure adhesion and compatibility.
There are also problems of resistance to scratching and abrasion, unwanted stiffness, even unwanted weight!
These finishes require spray booths and special equipment to apply, and since many are poisonous (even deadly) chemicals, they require operators to wear special protective gear, and breathe mechanically supplied fresh air.
Worse, they are atomized and sprayed into the air, so the majority of the costly and toxic finishing material is wasted, making a huge mess, and creating disposal headaches. Yikes!
Although guitar companies have their reasons for using these modern plastic coatings, I don’t think they belong on handmade guitars.
The traditional finish that I admire most is a two-component finish, a primer coat of some kind of protein (egg white? hide glue?), known in violin making as a “ground coat,” with a topcoat of terpene resin varnish.
The famed early violins from northern Italy wore these exceptionally handsome finishes, and have stood the test of time, still admired for their beautiful colors, depth, and richness.
I decided to use the essential ideas of this traditional two-part finish without risking a trip to the burn unit.
Instead of protein, I use one coat of epoxy resin, and instead of brushed on terpene resin varnish, I apply many padded coats of linseed oil as the varnish topcoat.
This reinterpretation of an oil varnish that I use for my acoustic guitars meets my design criteria:
Beautiful appearance. The finish must induce envy to be considered successful.
A protective film to exclude dirt and oils and slow the absorption of water vapor. I say slow, because no coating seems to be able to completely prevent water from somehow migrating through it into the wood.
Be thin, so as not to add significant weight or stiffness to the instrument. Some finishes on guitars are so thick that they act like reinforcing laminates!
Be easy to apply, touch up, repair and refresh. What good is a finish that looks trashed right away, or is a miserable challenge to repair?
Be safe to apply, and as non-toxic as can be. Guitarmaking has enough hazards already!
Be environmentally responsible. Of course, unless we can eat the finish, we can’t expect it to have zero toxicity or environmental impact, but we can try to be good citizens.
Here, I’ll show how this finish is applied.