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.

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at darrellnicholson.com.