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 in. to 0.040 in.), with most cases involving a nominal thickness of around 0.5 mm (0.020 in.).

Equipment: Gelcoat thickness can be measured with any of the following Panametrics-NDT thickness gages: Models 35 or 35DL. The waveform display that is available as an option on the 35 series, and that is standard on the advanced Model 38DL PLUS, is very useful for establishing echo quality and verifying readings.
Gelcoat is normally measured with a delay line transducer. The most commonly recommended Panametrics-NDT transducer is the M208-RM (20 MHz, 0.125 in. element). In cases involving thick gelcoat, the M202-RM (10 MHz, 0.25 in. 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 the 35-series waveform option or a Model 38DL PLUS is used, then it is possible to verify whether the first returning echo (representing minimum gelcoat thickness) is being detected, by monitoring the waveform display as seen in the screen images below. If a Model 35 or 35DL is used without the waveform display option, 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 in. element, high sensitivity). The maximum measurable fiberglass thickness will typically be approximately 12.5 mm (0.5 in.). For thicker fiberglass, a high penetration (HP) gage is usually required.