DeFelsko manufactures handheld, non-destructive coating thickness gauges that are ideal for measuring the dry film thickness of guitar finishes. The varnish is applied to the guitar to achieve a glass-like shine and accentuate the beauty and luster of the instrument. Although this article focuses on guitar varnish thickness, the same solutions can be used for polymer-based coatings on wood, plastic, graphite, and more.
Coating thickness measurement: challenges in measuring guitar varnish thickness
The primary challenge is to non-destructively measure the amount of lacquer applied to the surface of the guitar. A secondary challenge may be to also measure the amount of sealer (e.g. lacquer) applied to the substrate.
Dry Film Thickness Measurement Solutions
The standard-level Ultrasonic PosiTector 200 B1 is ideal for measuring total paint thickness + sealant thickness up to 1000 µm (40 mils).
Example of the standard-level model in storage mode
In addition to the overall thickness, the PosiTector 200 B3 (Advanced) can also individually identify additional layers, such as the sealant layer. The device is able to display the measurement result both numerically and graphically.
Example of the Advanced model in multi-layer mode
As shown above, the right side of the screen shows a graphical representation of the ultrasonic pulse reflections from the lacquer (polyurethane) and sealer (varnish) layers. The left side of the screen shows the numerical values for these layers, including the total composite thickness.
Background on guitar finishing
What is the application?
Varnish is applied to the substrate (usually wood or graphite) of the guitar to achieve a glass-like shine and highlight the beauty and lustre of the wood. The robust, durable finish protects the wood from moisture, chemicals and scratches, while optimising the acoustic resonance of the instrument.
Generally, manufacturers apply a series of coats of varnish or similar materials to the guitar to properly fill the wood. Some manufacturers apply coats of colour directly to the substrate and then apply further coats of varnish. Others use a combination of tinted varnish and clear coats. The thickness of each coat (or series of coats) varies depending on the individual finish and the processes used by the manufacturer.
Some common brand names of varnish include Polane® High-Solids Clear Topcoat and Sher-Wood® CAB-Acrylic (a two-part polyurethane varnish). Nitrocellulose lacquer is probably the most common finish used on older instruments. Many newer instruments are finished with polyurethane, catalysed lacquer, polyester and other high-tech finishes that are not solvent-based but catalysed.
Wood substrates for guitars include maple, mahogany, basswood, spruce, rosewood, alder, ash, walnut, cedar, and ebony. A common alternative to guitars with a wood substrate is graphite.Graphiteis a composite material made of carbon fibres in a resin matrix, usually epoxy. Although the acoustic properties of graphite are quite different from wood, both are equally suitable for measurement applications with paint coatings.
Why measure?
When applied in the wrong thickness, the coating that beautifies and protects a high-quality guitar can easily affect its sound. Too much varnish can dampen the acoustic resonance of the guitar, and too little can have the opposite effect.
The need to maintain guitar sound quality goes hand-in-hand with the need to control costs. Fast, non-destructive thickness measurements can be made over the entire surface of a guitar to ensure smooth, even coating without significantly disrupting the production process. Possible cost reductions include:
Minimising over-coating waste by controlling the thickness of applied lacquer
Minimising rework and repairs with direct operator feedback and improved process control
Eliminating the need to scrap guitars due to destructive coating thickness measurements
Companies that use destructive methods face many challenges that result in inaccurate, expensive and time-consuming measurements. To test destructively, the paint must be heated and a small piece cut into the surface. The still-hot piece must then be peeled from the substrate by hand and measured with a micrometer. This method often causes the coating to crack and warp, making it difficult to get an accurate reading with the micrometer. In addition, the guitars used for testing must be scrapped because the testing process damages them so severely that they can no longer be used. To obtain a statistically representative sample, several guitars from a batch may have to be scrapped as part of the destructive testing process.
Asmentioned in a PCI Magazine article, Gibson Measures Guitar Coatings Using Ultrasonic Technology, Gibson has been using the previous generation of the PosiTector 100 ultrasonic coating thickness gauge to measure the lacquer thickness on its wooden guitars since 1995. The gauge is now used hundreds of times a day in several Gibson plants.
Where is the market?
Customers interested in this particular application are primarily manufacturers and finishers of musical instruments based on wood or graphite. However, any manufacturer that coats wood products with varnish (e.g. flooring or furniture manufacturers) would benefit from the same cost control advantages.
Coating thickness measurement: challenges in measuring guitar varnish thickness
The primary challenge is to non-destructively measure the amount of lacquer applied to the surface of the guitar. A secondary challenge may be to also measure the amount of sealer (e.g. lacquer) applied to the substrate.
Dry Film Thickness Measurement Solutions
The standard-level Ultrasonic PosiTector 200 B1 is ideal for measuring total paint thickness + sealant thickness up to 1000 µm (40 mils).
Example of the standard-level model in storage mode
In addition to the overall thickness, the PosiTector 200 B3 (Advanced) can also individually identify additional layers, such as the sealant layer. The device is able to display the measurement result both numerically and graphically.
Example of the Advanced model in multi-layer mode
As shown above, the right side of the screen shows a graphical representation of the ultrasonic pulse reflections from the lacquer (polyurethane) and sealer (varnish) layers. The left side of the screen shows the numerical values for these layers, including the total composite thickness.
Background on guitar finishing
What is the application?
Varnish is applied to the substrate (usually wood or graphite) of the guitar to achieve a glass-like shine and highlight the beauty and lustre of the wood. The robust, durable finish protects the wood from moisture, chemicals and scratches, while optimising the acoustic resonance of the instrument.
Generally, manufacturers apply a series of coats of varnish or similar materials to the guitar to properly fill the wood. Some manufacturers apply coats of colour directly to the substrate and then apply further coats of varnish. Others use a combination of tinted varnish and clear coats. The thickness of each coat (or series of coats) varies depending on the individual finish and the processes used by the manufacturer.
Some common brand names of varnish include Polane® High-Solids Clear Topcoat and Sher-Wood® CAB-Acrylic (a two-part polyurethane varnish). Nitrocellulose lacquer is probably the most common finish used on older instruments. Many newer instruments are finished with polyurethane, catalysed lacquer, polyester and other high-tech finishes that are not solvent-based but catalysed.
Wood substrates for guitars include maple, mahogany, basswood, spruce, rosewood, alder, ash, walnut, cedar, and ebony. A common alternative to guitars with a wood substrate is graphite.Graphiteis a composite material made of carbon fibres in a resin matrix, usually epoxy. Although the acoustic properties of graphite are quite different from wood, both are equally suitable for measurement applications with paint coatings.
Why measure?
When applied in the wrong thickness, the coating that beautifies and protects a high-quality guitar can easily affect its sound. Too much varnish can dampen the acoustic resonance of the guitar, and too little can have the opposite effect.
The need to maintain guitar sound quality goes hand-in-hand with the need to control costs. Fast, non-destructive thickness measurements can be made over the entire surface of a guitar to ensure smooth, even coating without significantly disrupting the production process. Possible cost reductions include:
Minimising over-coating waste by controlling the thickness of applied lacquer
Minimising rework and repairs with direct operator feedback and improved process control
Eliminating the need to scrap guitars due to destructive coating thickness measurements
Companies that use destructive methods face many challenges that result in inaccurate, expensive and time-consuming measurements. To test destructively, the paint must be heated and a small piece cut into the surface. The still-hot piece must then be peeled from the substrate by hand and measured with a micrometer. This method often causes the coating to crack and warp, making it difficult to get an accurate reading with the micrometer. In addition, the guitars used for testing must be scrapped because the testing process damages them so severely that they can no longer be used. To obtain a statistically representative sample, several guitars from a batch may have to be scrapped as part of the destructive testing process.
Asmentioned in a PCI Magazine article, Gibson Measures Guitar Coatings Using Ultrasonic Technology, Gibson has been using the previous generation of the PosiTector 100 ultrasonic coating thickness gauge to measure the lacquer thickness on its wooden guitars since 1995. The gauge is now used hundreds of times a day in several Gibson plants.
Where is the market?
Customers interested in this particular application are primarily manufacturers and finishers of musical instruments based on wood or graphite. However, any manufacturer that coats wood products with varnish (e.g. flooring or furniture manufacturers) would benefit from the same cost control advantages.
