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Coating Thickness Measurement
of Lacquer on Guitars
Updated December 2007
DeFelsko manufactures
hand-held, non-destructive coating thickness gages that are
ideal for measuring the dry film thickness of lacquer on
guitars. 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.
 Measurement
Challenges
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.
Dry Film Thickness Measurement Solutions
The ultrasonic PosiTector 200 B/
Std (Standard) is ideal for measuring
total lacquer thickness + sealant thickness combined
up to 1000 μm (40 mils).
Example of Standard Model
in Memory Mode
In addition to total thickness, the PosiTector 200
B/ Adv (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.
Example of Advanced Model
in Multi-Layer mode
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.
Free Consultation
For
current pricing or to order these instruments, please contact
us by telephone (315) 393-4450, fax (315) 393-8471, or
e-mail techsale@defelsko.com. If you require additional technical information or have questions
relating to your particular application, we encourage you
to take advantage of our years of experience in recommending
the best gage for your application.
What is the Application?
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.
Why measure?
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:
- minimizing waste due to over coating by controlling the
thickness of the finish being applied
- minimizing rework and repair through direct feedback
to the operator and improved process control
- eliminating the need to scrap guitars due to destructive
coating thickness measurements
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 Magaizine 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.
Where is the market?
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.
PosiTector
200 |
