Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub     

Subject: Natural Marine Adhesives and Sealants — The Complete Guide 

Natural Marine Adhesives and Sealants — Casein Glue, Shellac, and Rubberised Bedding Compound for Plastic-Free Boatbuilding

The natural adhesive and sealant system for a skin on frame boat comes down to three materials. Casein glue takes the structural loads, shellac seals and primes and locks fastenings, and rubberised pitch compound beds hardware and pays seams. All three can be made in a kitchen or a small workshop from ingredients that have been available for centuries, and all three biodegrade cleanly at end of life. This hub pulls together the four guides that cover the system and sets out how the pieces fit.

The thing that surprised me most when I started building this way was how capable the adhesive system turned out to be. The assumption going in, mine and I think most people’s, is that you’re accepting a compromise. Natural is more principled but weaker, less durable, less waterproof. For the joints and sealing tasks a small craft actually presents, that assumption is largely wrong. A well-made casein joint is thermoset, doesn’t creep, and fails in the wood rather than at the glue line. Alcohol shellac seals end grain, locks fastenings, and primes timber, reversibly, with a solvent you could carry in a bottle the size of your thumb. Rubberised pitch compound beds through-hull fittings and pays seams with a compatibility polysulphide can’t match in a natural build.

How the four guides relate

Casein glue is the structural adhesive, the highest-strength material in the system and the one that bears the most direct structural responsibility. It handles load-bearing joints, frame members, lashing reinforcement, skin-to-gunwale bonding in some configurations. Treat it as the natural equivalent of a thermoset woodworking adhesive. Shellac is the most versatile material and the one I reach for most during a build. It seals timber and primes it for varnish, locks fastenings and seals screw holes against water ingress when applied in alcohol, and in ammonia-dissolved form gives a moderate bond for lightweight internal fittings and small hardware. Rubberised pitch compound is the bedding and seam material, thermoplastic, hot-applied, flexible when cured, self-sealing under compression, the natural equivalent of polysulphide or polyurethane bedding without the petroleum chemistry and with full reversibility.

They are compatible with each other and with everything else in a VAKA build, timber, linseed oil, pine tar, natural canvas, Le Tonkinois varnish. No component creates a compatibility problem with any other, which is one of the real advantages of working with traditional materials. They were developed alongside each other over centuries of use, and the interactions between them are well understood.

Casein glue, the structural adhesive

Casein glue is made from dairy protein activated with alkali, lime and potassium hydroxide in the VAKA formula, with copper sulfate for decay resistance and tannic acid to improve elongation to break. It is the primary structural bond for every joint in a VAKA hull. The properties that earn it that role are dry shear strength comparable to commercial woodworking adhesives, a thermoset cure that doesn’t soften under heat or creep under load, gap-filling viscosity suited to hand-cut joinery, and full reversibility with moisture and heat for future repairs. The casein-to-wood bond releases cleanly without damaging the timber, which in a hull meant to be maintained and repaired indefinitely is not a side benefit.

The Casein Glue: A Natural Wonder Glue post covers history, chemistry, working properties, and the comparison with PVA and epoxy, the case for using it as your primary structural bond. The How to Make Casein Glue guide has the VAKA formula in full, ingredients, ratios, the preparation sequence, and troubleshooting, covering both the powder route and the fresh dairy route, the alkali options including potassium and sodium hydroxide, and the role of each ingredient.

Shellac, sealer, threadlock, and adhesive

Shellac comes from the lac insect and has been used in woodworking, instrument making, and boat construction for centuries. In a VAKA build it does three jobs, each needing a slightly different formulation. As a sealer and primer, a dilute alcohol solution at a 1.5 to 2-pound cut on bare timber seals the grain, improves varnish adhesion, and closes end grain that would otherwise drink topcoats. As a threadlock, a denser solution at a 2.5-pound cut on the screw thread, driven wet, locks the fastening and seals the hole, reversibly, with methylated spirits or any high-strength alcohol. As an adhesive, shellac dissolved in ammonia rather than alcohol gives a moderate bond for lightweight fittings and small hardware.

One restriction carries through all of it. Alcohol shellac only near fastenings. Ammonia reacts with copper, bronze, and brass, causing stress corrosion cracking, and since almost all wooden boat fastenings are silicon bronze, brass, or copper, ammonia shellac belongs nowhere near them. The Shellac for Boat Building post is the complete guide to shellac as sealer, primer, reversible threadlock, and adhesive, with the pound-cut and metric reference table, the grades, dewaxing, shelf life, and the Herreshoff double-planking connection. The companion note on shellac as a waterproofer goes deeper on grades, solvents, and the waxed-versus-dewaxed question.

Rubberised marine glue, bedding and seam compound

The rubberised pitch compound is the hot-applied thermoplastic sealant for through-hull fittings, screw-hole bedding, and seam sealing over cotton caulking. It is made from brewer’s pitch, Stockholm tar, linseed stand oil, waxy shellac flakes, and unvulcanised natural crepe rubber dissolved in gum turpentine. It sits below casein structurally but handles work casein cannot: permanent flexible sealing where the hull keeps moving, compression sealing around metal fittings, and seam waterproofing over cotton. The natural rubber fraction gives it elasticity and snap-back under flex, and the linseed oil keeps it flexible through freeze-thaw, which is the failure mode of cheaper traditional pitch compounds that run too much shellac. It goes on hot to pre-warmed timber, so it penetrates end grain and fills voids around fittings in a way cold-applied sealants can’t. It is neither fast nor easy. It rewards patience and preparation.

The Traditional Rubberised Marine Glue and Bedding Compound post has the full recipe, the preparation method, and the application notes, with sourcing guidance and honest caveats about where the formulation has and hasn’t been fully tested.

Where each material goes in a build

To make the system concrete, here is where each one appears in a typical skin on frame construction. Casein glue takes all structural timber-to-timber joints, lashing reinforcement at frame junctions, gunwale-to-frame bonding, thwart knees, anything bearing direct structural load. Alcohol shellac at a 2-pound cut seals end grain on all cut timber before varnishing and primes all bare interior timber before Le Tonkinois. The same shellac at a 2.5-pound cut goes on all screw holes and fastenings before they’re driven home, deck fittings, fairleads, cleats, thole pins, any metal fastening into timber. Ammonia shellac bonds lightweight interior fittings and fabric trim and small hardware, where casein would be overkill and reversibility is the point. Rubberised pitch compound beds all through-hull fittings, beds any deck hardware that penetrates the skin, and seals the seams between the stems and the canvas skin.

The boat plans specify which material goes where at each stage for each design, and the full Field Notes cover the wider context, hull design, skin treatment, natural varnish, and the ecological reasoning behind building this way. The whole set lives under the Natural Sealants & Adhesives series and the broader Regenerative Materials collection.

At VAKA I design and build boats that don’t destroy the environment, one person, testing what works and writing it down honestly. The plans are at VAKA Plans as they come through sea trials, the research at Field Notes, and when there’s a boat to launch the Hithe Finder lists slipways, hards, and beaches for small craft. VAKA. Traditional craft and natural materials. Nottingham. 2026.