Simon Fay Guitars
After extensive experimentation with many different finish materials, I have settled on a urethane finish because I believe it is currently the best finish available for steel string guitars. I do all my own finish work and produce a remarkably thin finish of exceptional quality that rivals French Polish for tone.
For most of my building career (2008-2020), I outsourced my finish work to a talented (now retired) finisher who did an outstanding polyester finish for me. Over the years, I had few customers request a French Polish on the soundboard and I could not help but notice the tonal improvement this finish brought to my guitars. It is commonly cited that a thin finish is very important for guitar tone but my experience is that the finish type also has a noticeable effect as well. It is my opinion that the finish effect becomes much more noticeable on very responsive instruments. I believe the discussion of finish impacting tone is less important for mass-produced guitars so long as the finish is kept to reasonable levels — 5 to 8 mils thick. If you are unfamiliar with the term, a mil simply refers to one thousandths of an inch and thus, a 10 mil finish is 0.010” thick.
Finish characteristics that are important to luthiers:
Durability (Dent, Scratch, and Solvent Resistance)
Flexibility (Cold checking)
Effect on Tone
The major players here are shellac, nitrocellulose lacquer, oil varnish, conversion varnish, urethane, and polyester. Almost all production guitars and guitars from premier small shops and well-regarded luthiers utilize one of these finish materials.
French Polish (Shellac)
I love shellac. Tonally, it is the standard against which all other finishes ares measured. It has remarkable longevity and is truly an heirloom quality finish. It is a beautiful finish and imparts color depending on what type of shellac flakes are used. French Polish is a physically demanding and time consuming process that requires great skill to execute at a high level. It is quite challenging to produce a French Polish finish to the same degree of perfection modern steel string players are accustomed to seeing on their guitars. Personally, I enjoy the process but recognize it isn’t normally the best finish for some guitarists due to its delicate nature. One thing about shellac that many players may not be aware of is the wonderful tactile feel of the finish — French Polished necks feel incredible.
The biggest negative is that it is very delicate when new and some players will find that their perspiration quickly erodes the finish. It is easily scratched and offers little protection against marks and dents. If you own a French Polished guitar, it is helpful to have the mindset that you may need to have the guitar touched up every now and again.
Nitro is the traditional American guitar finish and is a hugely popular finishing material both among large manufacturers, small shops, and individual luthiers. Like shellac, it is easily repaired but also much less labor intensive. The finish is beautiful and it is relatively easy to apply a very thin finish. If applied at or under 3 mils, lacquer will yield an exceptionally tone friendly finish that rivals both shellac and oil varnish. The biggest negatives are that it scratches easily, cold checks, has very poor solvent resistance, can be damaged by the plasticizers in rubber materials, has a high friction feel (gloss necks are “grippy”), and begins to yellow and decay after several decades.
A thin oil varnish rivals French Polish for tone. It is easy to buff to a mirror gloss, doesn’t cold check, and has much better solvent resistance than lacquer. Like shellac, varnish can last for a very long time and has an equally wonderful tactile feel. It is roughly equivalent to nitro in terms of dent and scratch resistance. Unfortunately, it has poor repairability. For the most part, I was extremely happy with my oil varnish results but struggled with drying issues over Rosewood and application issues (surface tension pulling the finish away from edges). I’m OCD about my work and I just wasn’t able to get the degree of perfection I desire in my finishes. However, the biggest issue for me was the noticeable amber cast the finish imparts to the guitar. Oil varnish looks absolutely spectacular over Mahogany but I didn’t particularly like how it looked over Spruce, Maple, and very dark woods like Ebony.
Polyester is the finish of choice for a lot of large production manufacturers. Kevin Ryan and Jim Olson and a number of other luthiers use polyester finishes on their instruments. It is a two component finish that catalyzes via chemical cure or a UV cure. Polyester has outstanding physical properties. It is amazingly durable and has phenomenal scratch, dent, and solvent resistance and does not cold check or decay. Even industrial strength paint strippers are ineffective on polyester finish. The tactile feel of a gloss material is excellent but not quite as “silky” as shellac or oil varnish. In my opinion, it is a superb choice for factory built guitars. The finish can be applied as a very thin film but is typically applied as an overly thick film when used on low cost instruments. It is important to understand that the skill with which the finish is applied is often more important than the finish type. For example, a thick 10 mil French Polish finish would negatively affect tone.
As I mentioned above, I used polyester on my guitars for many years and absolutely loved the durability and appearance of the finish. My finish guy did a spectacular job and applied a thin 3 to 5 mil finish on the top. Unfortunately, I alway noticed a difference in the sound of my guitar pre and post finish — with the latter sounding more constrained and less open. A number of us in the lutherie community have come to the conclusion that polyester “eats tone”. Please note that this effect on tone can be significantly minimized by applying the polyester finish as thin as possible on the soundboard. The repairability of polyester is similar to oil varnish and urethane. It is challenging to perform invisible spot repairs like one can with lacquer or French Polish.
Urethane is a two component finish like polyester but it is a softer and more flexible coating. It is closer to nitro than it is to polyester in terms of hardness but still offers significantly better scratch, dent, and solvent resistance than lacquer. It is certainly hard enough that it doesn’t get scratches from general handling the way that nitro, shellac, and varnish do. Urethane doesn’t cold check and it has excellent solvent resistance - only very strong chemicals can affect it. The tactile feel is superb and equally as nice as varnish and shellac. Longevity is also excellent and the finish doesn’t degrade over time. Repairability is similar to varnish and polyester. Aesthetically, urethane is my favorite finish due to its remarkable clarity. This clarity is actually a detriment on thick finishes which yield a miles deep, “dipped in plastic” look that looks cheap and detracts from the beauty of the woods. However, a reasonably thin urethane finish looks absolutely breathtaking. The clarity acts as a lens that magnifies grain and causes woods to burst with color and shimmer under light. And most importantly, an ultra-thin urethane finish (3 mils or less) is similar to shellac in what it does for tone - or to be more specific, what it doesn't do to the tone. My guitars pre-finish and post-finish sound almost identical with a 3 mil urethane finish.
For ultra-thin finishes (3 mils or under): shellac, varnish, nitro, and urethane all become low percentage variables that have very little effect on tone. At this level, the finish is thin enough that a guitar pre and post finish will sound nearly identical. For the absolute best tone, one of these finish materials at this coating thickness is what you want on your guitar. Oil varnish and shellac are relatively easy to keep at 3 mils or under and I believe this is the primary reason these finishes sound so good. However, a film thickness this thin in lacquer or urethane is exceedingly rare even in the high-end boutique and luthier-built market.
In the context of an ultra-thin finish coating on a responsive guitar, I believe urethane stands above the competition. It is the only finish that truly offers excellent durability and doesn’t negatively affect tone. I also give urethane very high marks on appearance and tactile feel. The only downside to a urethane finish is that it can be challenging to repair.
I’d also like to take the time to discuss two popular myths regarding nitrocellulose lacquer.
No other finish lets the wood breathe like nitro.
First of all, finishes don’t breathe nor is nitrocellulose similar to the cellulose in wood. Nitrocellulose is in the same chemical family as nitroglycerin and TNT. It is far removed from the cellulose in wood and is highly explosive and flammable in high concentrations. The most natural of all the finish resins would be shellac which is refined from secretions from the female lac bug. Modern oil varnishes are usually made with phenolic or urethane resin and are no more natural than nitro, polyester, or urethane. Homemade oil varnishes used on violins are quite different from the varnish used on guitars or furniture and use naturally occurring components such as amber, copal, shellac, sandarac, mastic, and linseed oil. Chemically, all of these finishes try to accomplish the same thing — deposit a protective resin over the wood that prevent moisture, dirt, and chemicals from entering the wood as well as provide durability. None of the finishes used on guitars allows them to “breathe”.
When looking at finishes it is vitally important to understand that whether the resin is man-made or naturally occurring, all of these resins are just polymers (either thermoset or thermoplastic). Synthetic resins have the added property of being able to be manipulated to achieve a wide range of physical characteristics and this is a huge advantage for finish materials and why modern finish coatings can be tailored to have specialized characteristics for specific industries (marine, indoor, outdoor, musical instruments, etc.)
Poly finishes look like plastic and don’t sound as good as lacquer.
The modern poly finishes (urethane and polyester) are often touted as being thick, less attractive finishes that don’t let the wood vibrate like a good nitrocellulose lacquer finish. The reality is that this statement is typically only true when directed at entry-level, mass-produced guitars. However, any of the top-tier guitar manufacturers like Martin (nitro), Taylor (polyester), and Larrivee (urethane) produce reasonably thin finishes on their guitars. Tonally, I'd choose nitro or urethane over polyester but neither of these finishes offer the kind of bulletproof durability that polyester provides.
Regarding appearances, all three finish materials look very similar. I don't think there is a person alive that could identify whether a finish is lacquer, urethane, or polyester just by looking at it. Subtle differences would be noticeable if you applied the coatings adjacently on a piece of wood but even then, most folks would have difficulty telling them apart. Oil varnish and shellac are more readily distinguished but that is mostly on account of the color those materials impart to the surface. All of these finish materials can produce beautiful results when they are combined with a good sealer and pore filler that pops the grain and brings out the wood's color.
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