Testers Check Epoxy Sandability, Adhesion, and More


Testers evaluated sandability, resistance to sag, cure time, and adhesion using sample fiberglass panels with 3-by-3-inch test swatches. The fairing compounds were applied and cured in temperatures ranging from the low 60s to the mid-80s. Each test was repeated three times, and the results were averaged. The shaping test involved closely timed periods of even sanding using a block sander and new sheets of 80-grit sandpaper. Testers observed each materials tendency to clog the sandpaper, and measured the volume of accumulated dust after four minutes of sanding.

pencil kit

To evaluate hardness, testers created several molded sample coupons of each filler. A Gradco pencil hardness test kit was used to quantify the findings. The hardness testing utilized five different lead grades (H6, H3, H, B3, and B5) to determine the relative hardness of each epoxy filler.

The molded coupons were also used to measure adhesive ability. To test adhesion, we applied each filler to an inverted cup mold. The mold had been washed in order to degrease the surface, but there was no additional effort to sand or prep the surface. The assumption was that the inherently poor adhesive nature of the thermoplastic would result in a very poor bond. The adhesion test involved both dry tests and submersion testing in hot water. As expected, some of the coupons lost their grip on the cups while others remained adhered throughout.

Finally, each inverted cup was dislodged using a large, wide-bladed chisel, which provided some interesting subjective insight into the relative adhesive strength of the various epoxies.

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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