DeFelsko manufactures hand-held, non-destructive coating thickness gages that are ideal for measuring the dry film thickness of lacquer on guitars. Lacquer is applied to the guitar to attain a glass-like sheen while showing the beauty and luster of the instrument. Although this article focuses on lacquer thickness applied to guitars, the same solutions can be applied to polymer based coatings on wood, plastic, graphite and more.
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 sealant (e.g. varnish) applied to the substrate.
The ultrasonic PosiTector 200 B1 (Standard) is ideal for measuring total lacquer thickness + sealant thickness combined up to 1000 μm (40 mils).
In addition to total thickness, the PosiTector 200 B3 (Advanced) instrument is also capable of individually identifying additional layers such as the sealant layer. The instrument is capable of providing both numerical and graphical representations of the measurement result.
As shown above the right side of the screen presents a graphical representation of the ultrasonic pulse reflections of both the lacquer (polyurethane) and sealant (varnish) layers. The left side of the screen provides the numerical values corresponding to these layers, including the total thickness summation.
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Lacquer is applied to the substrate (typically wood or graphite) of the guitar to attain a glass-like sheen while showing the beauty and luster of the wood. The tough durable finish of the lacquer protects the wood from moisture, chemicals and marring, while optimizing the acoustic resonance of the instrument.
In general, manufacturers apply a number of layers of lacquer or similar finishing materials to the guitar to get the wood filled properly. Some manufacturers apply layers of color directly to the substrate, and then apply additional layers of lacquer. Others use a combination of tinted lacquer and clear coats. The ability to identify the thickness of individual layers (or series of layers) is dependent on the individual finishes and processes used by the manufacturer.
Common brand name lacquers include Polane® High-Solids Clear Topcoat and Sher-Wood® CAB-Acrylic (a two-component polyurethane coating). Nitrocellulose lacquer if probably the most common finish used on older instruments. Many newer instruments are being finished in polyurethane, catalyzed lacquer, polyester and other hi-tech finishes which are not solvent based, but rather catalyzed finishes.
Wood substrates for guitars include maple, mahogany, bass, spruce, rosewood, alder, ash, walnut, cedar, and ebony. A common alternative to wood substrate guitars is graphite. Graphite is a composite material of carbon fibers in a resin matrix, usually epoxy. Though the acoustic properties of graphite are significantly different from wood, both are equally suited for measurement applications involving lacquer coatings.
When applied at the wrong thickness, the coating that beautifies and protects a high-quality guitar can easily detract from its sound. Too much coating can dampen the guitar’s acoustic resonance, and too little can have the reverse effect.
The need to maintain the sound quality of the guitar is combined with the need for controlling costs. Quick non-destructive thickness measurements can be taken over the entire surface of a guitar, ensuring a smooth even coating, without significantly disrupting the production process. Potential cost reductions include:
Companies utilizing destructive methods face many challenges resulting in inaccurate, expensive and time-consuming measurements. To destructively test, the Lacquer must be heated and a small piece cut into the surface. While still hot, the piece has to be stripped from the substrate by hand and measured with a micrometer. This method may often tear and distort the coating, making it difficult to measure accurately with a micrometer. In addition, the guitars used for testing need to be scrapped because the testing process damages them beyond usable condition. To get a statistically representative sample of the process several guitars from a lot may need to be scrapped as part of the destructive testing process.
As referred to in a PCI Magazine article, Gibson Measures Guitar Coatings Using Ultrasonic Technology, Gibson has been using the previous generation PosiTector 100 ultrasonic coating thickness gage to measure lacquer on their wooden guitars since 1995. The gage is now used hundreds of times per day in several of Gibson’s facilities.
Customers interested in this particular application would primarily be manufacturers and finishers of wood or graphite based musical instruments. However any manufacturer that coats wood products with lacquer (i.e. flooring or furniture manufactures) would also experience the same cost control benefits.