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 Application: Measuring the thickness of gelcoat over fiberglass.
Background: Gelcoat is a hard polyester resin coating that is applied over structural fiberglass to provide a smooth, glossy protective surface that eliminates friction and improves appearance. It is most commonly used on fiberglass boat hulls and on bathroom fixtures such as sinks, tubs, and shower stalls. Manufacturers of fiberglass products need to measure gelcoat thickness to insure that it is within a specified range, and especially to detect situations where
it is too thin because of insufficient application or shrinkage. Measurements typically involve a range of approximately 0.25 mm to 1 mm (0.010 inch to 0.040 inch), with most cases involving a nominal thickness of around 0.5 mm (0.020").
Equipment: Gelcoat thickness can be measured with any of the following Panametrics-NDT
thickness gages: Models 35 or 35DL. The waveform display of the Model 25DL PLUS is very useful for establishing echo quality and verifying readings, but it is generally not required.
Gelcoat is normally measured with a delay line transducer. The most commonly recommended Panametrics-NDT
transducer is the
M208-RM (20 MHz, 0.125" element). In cases involving thick gelcoat, the M202-RM (10 MHz, 0.25" diameter element) can also be used.
Procedure: This is set up as a Mode 2 measurement (interface echo to first backwall echo). The primary challenge is that the inside surface of gelcoat is usually rough and irregular, conforming to the surface profile of the fiberglass, and because of this non-uniform geometry the quality of the echo from the gelcoat/fiberglass boundary often varies widely from point to point. The first echo from that boundary may not be the largest, phase distortion may appear to
invert echoes, and in some cases roughness may drop reflected echo amplitude below the detectable minimum. Also, the gelcoat/fiberglass boundary echo is usually followed by several other peaks representing scatter noise from within the underlying fiberglass, which if detected will cause erroneously high readings.
Because manufacturers are usually most concerned about insuring that gelcoat meets a minimum thickness specification, the recommended procedure is to make several readings within a small area and record the minimum reading obtained. If a Model 25DL PLUS is used, then it is possible to verify whether the first returning echo (representing minimum gelcoat thickness) is being detected, then by monitoring the waveform display as seen in the screen images below. If a
Model 35 or 35DL is used, it is particularly important to note the minimum reading within a local area. Any abrupt transition to a higher reading usually represents false detection caused by irregular echoes. As a general rule, gage gain should be set high to maximize the likelihood of detected the first returning echo.
Optimal gelcoat reading - first peak detected
Inaccurate reading - first peak not detected due to low amplitude related to inside surface roughness
Note: Measurement of total fiberglass thickness
In many cases, the same gage that is used for gelcoat measurement can also be used to measure total wall thickness (gelcoat plus fiberglass) using a second transducer, usually an M1036 (2.25 MHz, 0.5" element, high sensitivity). The maximum measurable fiberglass thickness will typically be approximately 12.5 mm (0.5 inch). For thicker fiberglass, a high penetration (HP) gage is usually required.
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