Oiling Plywood

Wood encapsulation is an almost universal principle in modern boatbuilding. As amateur builders, we feel reassured when our labor of love can be encased in a protective plastic barrier. It just seems wrong to spend hundreds of hours building a boat out of wood only to toss it into water like a sponge into the sink—yet ancient boatbuilding techniques worked with, not against, water absorption.

For seafaring boats of the Pacific Northwest, the enemy is rain rather than salt water. With approximately 80″ of rain per year, freshwater pooling in wooden boats is a microbe haven whereas salt water, in this relatively cool climate, is inherently a preservative. Here, unless a wooden boat is hauled into a shop, it will be bathed in rain or water vapor for eight to nine months per year, and tarping it can spell disaster. Coverings wrapped tightly over the gunwales trap humidity like a greenhouse, promoting new cracks in the wood and fungal stains across the brightwork, under the varnish.

To Coat or Not to Coat

It’s natural to assume that paint or varnish, or especially a few coats of epoxy, will waterproof wood. And to an extent, this approach does work for lightly used pleasure boats. Full encapsulation is inherent in a stitch-and-glue boat where plywood planking has fiberglass-and-epoxy protection inside and out. But for the traditional plywood-on-frame design, ’glassing an interior is unrealistic, and a boatbuilder will struggle to apply a barrier that penetrates all the cracks and crevices created by extensive and awkward framing joints. Once a boat is complete and in use, a proliferation of unseen dings, scratches, rubs, and pokes will perforate a coating of paint, varnish, or epoxy-varnish.

Photographs by the author

Applying water-barrier protection such as ’glass and epoxy to a complicated interior structure is not always practical. For working boats like this Pacific City dory, applying boat soup on an annual basis is an affordable and long-lasting approach. The bilges of KAPETAN, seen here, were treated when new and have been re-treated annually for the past 10 years.

In the Pacific Northwest, many ply-on-frame boats are built of fir marine plywood on fir framing. The materials are easy to work, widely available, and more or less affordable. But fir is soft, and a varnish- or paint-over-epoxy coating is vulnerable due to the softer underlying wood matrix that provides less support to the thin barrier layer. The day-to-day grind will, one way or another, penetrate the coating and, once entrance points are created, water creeps in and under.

Untreated, Douglas fir is rot resistant and can withstand countless cycles of absorption and drying. But around a small puncture a barrier coat will trap water and prevent normal cycles of evaporation.

“Poison It and Oil It”

The life of an Oregon beach-launched semi-dory (or drift boat, on Oregon’s coast-range rivers) is about as hard as it gets, and traditional-boat owners in the region long ago gravitated to a simple approach to preserving the life of their craft: “Don’t paint it, don’t epoxy it, don’t varnish it—poison it and oil it.”

Ideally, the treatment is first applied to bare wood surfaces, when the boat is new, but it can be applied whenever there is bare, untreated wood to be protected.

An important trick when applying boat soup is to warm the boat so that the oil retains its low viscosity, reaching into nooks and crannies, and penetrating deep into the plywood to achieve maximum saturation. A warm sunny day can often provide enough heat to make a significant difference.

Since an oil-and-wood matrix can, by itself, be a beneficial environment for some microorganisms, the local prescription is first to saturate a boat’s interior plywood and framing with an antimicrobial/antifungal preservative—to “poison it.” I use the iodine-based (iodopropynl butylcarbamate, IPBC) Woodlife Classic Clear. Next, once the wood is completely dry, the boat is warmed in the sun or a heated shop to prepare it for oiling. (The warmer wood temperature will help to maintain the oil’s lower viscosity as it penetrates, and also create a temperature gradient that pulls the oil inward.)

Now the boat is ready for the “boat soup” to be applied.

What’s in the Soup?

Boat soup refers to a variety of recipes that include oils and turpentine. In my region, it is commonly a mix approximating 45% turpentine, 45% raw linseed oil, and 10% pine tar and/or tung oil, or similar. Raw linseed oil penetrates far better than boiled, which tends to cure like a varnish, an outcome fundamentally at odds with the goal of oiling plywood. Pine tar promotes a darkening of the plywood over time until, after decades of use, it is essentially black, while tung oil only accentuates the wood grain’s natural color. The boat soup has a low viscosity allowing it to penetrate beyond the surface grain to provide protection while still allowing the wood to breathe.

How to Apply the Boat Soup

Typically, the mix is applied with a large brush. Working on bare wood, a dory owner may apply several coats, requiring two gallons or more for a new 22′ dory’s interior. After this first multi-coat application, the grain begins to fill, and thereafter usually requires only one coat per season.

In damp conditions, such as those experienced in the Pacific Northwest where annual rainfall on the coast can exceed 80″, wrapping a wooden boat in a waterproof tarpaulin can trap humidity in the boat’s interior. To avoid this, it is often better simply to leave the boat uncovered and tilted so that rainwater flows out through an open drain plug.

Does It Hold Up?

Of the oiled Pacific dories I have examined, interior plywood that has been treated with boat soup generally looks remarkably clean for its age, with limited or no checking. In contrast, plywood surfaces that have been painted, varnished, or even epoxy-coated, show significant checking. My assumption is that low viscosity, non-curing boat soup allows the plywood to go through countless temperature and moisture cycles more evenly, in a way that supports the stability of the plywood’s surface layer, and thus mitigates checking. Some local dorymen have suggested that the quality of the plywood also plays a significant role. True AA marine fir plywood would seem to be a thing of the past, and most modern AB plywood does seem to check more easily. Nevertheless, boats whose wood has been saturated with numerous coats of soup early, show significantly less checking and any checking that does appear, appears later in life.

As I enter my sixth season in the Pacific City Dory fishery, my appreciation for this low-tech approach to wood protection has only grown—for its simplicity, its aesthetics, and its longevity. Scores of 50-year-old Pacific City Dories are still in regular use with beautiful, oiled plywood interiors—a tried and tested local adaptation to a challenging environment.

John Goodell is a museum director and wildlife biologist. In 2019 he built his boat, TSHAWYTCHA, a 23′ Glen-L Hunky Dory. He oiled her interior when new, and has continued to renew the oil treatment every year since. He has worked TSHAWYTCHA in the Pacific City Fishing Dory fleet since her launching.

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