We have spent some time with plywood boats over the last several years, both vintage and new, and a common problem that we have had to address is cracks that can develop in the outer veneer of the plywood. When plywood is made, the wood fibers are stressed from their original shape to form flat veneers, and over time the wood dries out and the fibers shrink. Repeated wet-dry cycles and sun exposure also can cause damage to veneer protected only by paint or varnish. The cracks, also referred to as checking, can be very fine or larger fractures that form a rough surface. At worst, there can be voids where bits of veneer have chipped off.
Applications of epoxy, both straight and thickened with silica, have been very effective in repairing the plywood and preventing further checking. Epoxy has adhesive and sealing properties that are superior to those of paint. Applied to bare wood, epoxy stabilizes the grain and provides a protective barrier to water intrusion. It provides a solid base for subsequent applications of varnish or primer and paint. If the surface being restored is especially rough, as it can be with fir plywood, fairing compound can make it smooth.
We built our Penobscot skiff with okoume plywood five years ago. The instructions provided by designer Arch Davis prescribed two coats of epoxy to seal the grain of the veneer before applying fairing compound to smooth the irregularities caused by the plywood and fastener holes; primer and polyurethane paint on the hull; and alkyd primer and paint for the topsides. The plywood has not checked and the finish is still smooth.
Arch’s advice got us thinking about using epoxy on a 1953 Alcort Sunfish. The painted fir plywood deck had extensive checking. We sanded it back to bare wood and applied two coats of WEST System 105 Resin with 207 Special Clear Hardener, which sealed the grain and gave the deck a nice bright finish. That deck has not checked since 2013. We store the boat in our garage when not in use, so there was no need to varnish over the epoxy to protect it from the sun’s UV rays.
We also used epoxy to treat two plywood Alcort Sailfish. The Standard Sailfish had the original 1950s deck and hull panels, which we sanded to bare wood; the Super Sailfish got new plywood for the bottom. It was easy to roll two coats of epoxy on the new plywood as well as on the sanded original panels, sanding between coats. Both Sailfish were to be painted, so we could apply an epoxy-based fairing compound and sand the surface smooth before priming and painting. There has been no checking on the old or new plywood since 2016 and 2019, respectively. The Standard Sailfish was painted with alkyd enamel, and the Super Sailfish was painted with a one-part polyurethane. A wide variety of paints can be used as long as they are applied over a good epoxy-compatible primer.
For small cracks and larger voids—bits of veneer that have come loose—we have used TotalBoat’s THIXO (epoxy thickened with silica) to fill the voids and provide a smooth surface for fairing compound, primer, and paint. The THIXO can be applied over sanded, clean surfaces, so it was not necessary for us to remove well-adhered paint to get to bare wood. We have not had checking through the silica-thickened epoxy either; it is stronger than straight epoxy.
Application of epoxy is just as easy as painting and restores checked plywood to like-new condition. It is not necessary to apply fiberglass cloth unless cloth is desired for specific high-wear areas. And, if you’re building a new boat, coating the plywood with epoxy before painting or varnishing is a step well worth the time and expense.
When not treating checked plywood, Audrey and Kent Lewis mess about in their armada of 16 small boats. Their adventures are logged at www.smallboatrestoration.blogspot.com
For the editor’s perspective on checking and his repairs inspired by this article, see “Checking My Work” in this issue.
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