As a general rule, any kind of finish on a guitar will have the effect to stiff the external fibres of the wood increasing the dampening
factor and raising the resonance point of the instrument compared to an unfinished one.
So it comes: the best finish is the one which allows the wood to behave just like it si not finished.
The result is that an unfinished instrument will always play better than a finished one (please read the sound of woods and the tapping method
Finish has the very important role to protect the instrument though as it prevents it to get dirty and oxidize due environmental temperature and moisture variations which will cause structure and stability problems. Without the finish protection, very clear coloured woods like maple for example, would soon display a dark brownish/greenish unpleasant colour especially on the neck where the hands run.
Furthermore woods tends to rapidly absorb and release water following the change in the atmospheric conditions. An unfinished neck would consequently force us to repeated truss rod adjustments.
If the kind and finishing procedure has a very deep impact on acoustic instrument's sound boards (usually in spruce), it also has some impact on electric instruments especially on the neck.
Leaving apart very tricky finishes procedures like shellak mostly employed in acoustic classical instruments like violins, etc, the following are the most diffused types of finish in hardness order:
Each of this finishes will have a different impact on who the instruments vibrates and therefore on its sound.
|A Frudua GFF-MIX quilted maple contoured top after aniline treatment
covered by the first sealer coat.
The use of essential oils for finishing an instrument requires, to be done properly and achieve finest results, very high skills and long applying experience. If applied generically this material will be deeply absorbed by the wood depth and never really dries causing a dramatic dampening of the vibration and the tone and reducing brightness and liveliness.
Though oils offers a minimum of protection from environmental changes it doesn't offer ad adequate protection from oxidation and a neck will rapidly get dirty due to sweat with no possibility of being reconditioned. We therefore recommend oil finishes only to for experts who know how to apply so that the final result is a very thin and hard dried coat.
Nitro is most probably the best treatment you can apply to you instrument. Along time this kind of finish releases and lose the greater part of its solvents getting thinner with a double advantage of getting stiffer and harder (dampening factor reduction) and reducing its impact on the tone getting more similar to a unfinished instrument.
This is the reason for some of the old nitro finished guitars tone is improving along time and are so sought after.
On the other side a nitro finished instrument will be slightly less stable and prone to move due to environmental climatic changes compared to another one finished with urethane or acrylic and it will also be more sensitive to mechanics shocks (scratches, bumps, etc).
Catalyist-fitted finishes (urethane/acrylics)
These are some of the most used finishes on guitar making. They are made of two components the finish itself and a catalyst which ensure a proper drying. If sprayed properly this these kind of finishes offer the most compromise between tone and protection and this is the reason they are so diffused. They dry fast and stabilize rapidly offering a good protection against scratches and bumps. Urethane tends to yellowish when aged while acrylics usually don't. Acrylics are usually associated to a very transparent look.
These are made of the finish a catalyst and a hardener. Polyester finishes are very diffused on cheaper instruments due to the easy applications and are very dangerous for health. Although they dry very fast and and harden almost like a glass due to their composition they are usually sprayed in thickness and consequently have a deep impact on tone for the reason explained above.
These are half way between polyester and urethanes. They dry hard but doesn't force you to very high thicknesses and are more transparent and clear than urethanes. These kind of finish is typical of USA market and usually applied in combination with pore fillers to reduce thickness.
Ultra Violet finished are two components materials which react to ultraviolet light by curing in a few minutes. They allow very fast finishing times but require very expensive owens to be used, which makes them suitable only for those companies who have a lot of instrument to build and finish. Water base finishes
Water base finishes are mono component materials very common in USA but still few diffused in Europe. European produced water based finishes tend to always remain a bit softer, like it never really dries completely which makes it unpleasant for some. They have pretty long curing times. BEHAVIOUR AND AGING
Finish changes its structure while drying (especially Nitro) and also at the change of atmospheric conditions.
The thicker layer of finish usually found on modern instruments makes them more sensitive especially to temperature changes.
Some of us may have noticed a change in the tone of our instrument along the day (from the afternoon to the evening), especially during very hot days. if your instrument (especially the neck) is finished with a thick layer of finishes as happens on most modern guitars, the finishing structure may have slightly changed along the day especially if you pass for example, from the very hot external temperatures to heavily conditioned environments. This may influence the resonance especially on the neck and lead to hear variations in the instrument's tone.
Same may happen in very cool days when outside temperatures are very low and while inside air is very hot and dry.
This shows how it is very important to finish (and glue as well) in controlled temperature and moisture contents environment to avoid moisture being trapped inside the finish.
In my experience, if you can't afford very expensive UV finish ovens, the best compromise for finishing is 3/4 thin coats of urethane sealer which is left to dry long for some days then finished with 2 coats of very thinned satin urethane finish 20 gloss. This kind of finish represents the best compromise I have ever found between a proper protection and handling and a fine tone.
The below video shows how to make you neck much faster with a simple hint you can always recondition.
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