Quote:
Originally Posted by LionEyes
Are there very many guitar makers that use a urethane finish?
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I have used catalyzed urethane for the last 8 years. Prior to that I used nitro. Going from nitro to Cat U was one of the best moves I have ever made in regard to finishes. It is very durable yet flexible enough to move with the wood. {If} applied thinly it has not negative impacts on tone.
I should point out that one should not use the word POLY as a generic term because not all polys are equal or even comparable. Poly-ester finishes are what some of the OEMs are using and Taylor comes to mind. Its tricky to apply and the finish builds very VERY quickly. If you don't have much experience shooting it the finish can be applied EXTREMELY thickly because the finish has a much higher solids content. Thick finishes can kill the tone of a guitar but the plus side is the finish is as hard as concrete so its almost like armor plating the instrument. Polyester will not scratch easily nor will it dent as easily as a softer finish.
Poly-urethane is one step down the hardness scale from polyester. It offers good scratch resistance, moderate dent resistance as well as resistance to most chemicals on the planet.
Instrument grade finishes are NOT the same finishes that one can purchase at their local home improvement store. Just don't think that you can go down to Lowe's and pick up some polyurethane, furniture lacquer or spar varnish and expect it to work on a musical instrument.
Nitro is the next down the hardness scale.
Varnish is among the softest finishes that I have tested to date and I have been on a testing rampage as of late. My urethane product, that I had used, is no longer available because the manufacturer went out of business last year so I have been trying to find a suitable replacement. Varnish is one of the slowest finishes to reach full hardness and one must allow ample time between spraying sessions and prior to buffing. Varnish has good resistance to scratches but poor resistance to dings and dents. It can become temporarily softer if subjected to intense heat (ie left in a HOT car) but will regain its hardness after it cools. In my own tests down to 2*F varnish has not exhibited any signs of cold checking.
Varnish can react with rubber and plastics but seems less susceptible after it has cured for 30 days. Learned this the hard way ;( Some say that Varnish damps the tone but I have not noticed this on the instruments that I have tested it on. Some, but not all varnish, is photoreactive to UV light. It can yellow with age similar to nitro. Most acrylic varnishes have UV inhibitors and are water white while most short oil alkyd varnishes do not and many of those are more amber in color.
I have fallen in love with the rich organic look of short oil varnish. It seems to cosmetically enhance the beauty of wood like no other finish that I have experienced. It has a depth and clarity to it like no other. Sheen seems to vary from brand to brand as does final hardness and drying / cure times. The plus side is that it can be applied as thin or thinner than any finish I have tested. Its not impossible to end up with a final finish in the 1.5 to 2.5 mils range. On sample spruce test panels I was unable to measure the thickness of 3 coats with a micrometer! Crazy as it sounds but true.
Bottom line is there are no ideal perfect finishes. They all have their own unique idiosyncrasies that one must learn to deal with and share with the end user.