The Guide to Natural Marine Adhesives & sealants

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub     

Subject: Natural Marine Adhesives and Sealants — The Complete Guide 

Field Entry: April 12, 2026

One of the things that surprised me most when I started building with natural materials was how capable the adhesive system turned out to be. The assumption going in — mine, and I think most people's — is that you are accepting a compromise: natural is more principled but less strong, less durable, less waterproof. In practice, for the joints and sealing tasks that a skin on frame small craft presents, that assumption is largely wrong.

A well-made casein glue joint is thermoset, does not creep, and fails in the wood rather than at the glue line. Alcohol shellac seals end grain, locks fastenings, and primes timber for natural varnish — reversibly, with a solvent you can carry in a bottle the size of your thumb. Rubberised pitch compound beds through-hull fittings and pays seams in a way that polysulphide cannot match for compatibility with a natural construction system. All of them can be made in a kitchen or a small workshop from ingredients that have been available for centuries. All of them biodegrade cleanly at end of life.

This hub brings together the four guides that cover the complete natural adhesives and sealants system used in VAKA construction, and explains how they fit together.

The System and How the Four Materials Relate

The four materials cover different parts of the bonding and sealing task:

Casein glue is the structural adhesive. It handles load-bearing joints — frame members, lashing reinforcement, skin-to-gunwale bonding in some configurations. It is the highest-strength material in the system and the one that bears the most direct structural responsibility. Think of it as the natural equivalent of a thermoset woodworking adhesive.

Shellac does three distinct things: it seals timber surfaces and primes them for natural varnish; it locks fastenings and seals screw holes against water ingress when applied in alcohol solution; and in ammonia-dissolved form it provides a moderate-strength adhesive bond for lightweight internal fittings and small hardware. It is the most versatile material in the system and the one I use most frequently on a day-to-day basis during a build.

Rubberised pitch compound is the bedding and seam compound. It handles through-hull fittings, deck hardware bedding, and seam sealing over cotton caulking. It is thermoplastic, hot-applied, flexible when cured, and self-sealing under compression. It is the natural equivalent of polysulphide or polyurethane bedding compound — without the petroleum chemistry and with full reversibility.

They are fully compatible with each other and with every other material in VAKA construction: timber, linseed oil, pine tar, natural canvas, Le Tonkinois varnish. No component in the system creates a compatibility problem with any other. This coherence is one of the genuine 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 make it the right choice for this application: dry shear strength comparable to commercial woodworking adhesives, thermoset cure that does not soften under heat or creep under sustained load, gap-filling viscosity that suits hand-cut timber joinery, and full reversibility with moisture and heat for future repairs. The casein wood bond releases cleanly without damaging the timber — which, in a hull designed to be maintained and repaired indefinitely, is not a peripheral advantage.

The overview post covers history, chemistry, working properties, and how it compares to PVA and epoxy. The how-to guide covers the recipe and mixing procedure in detail.

Casein Glue: A Natural Wonder Glue — Properties, history, comparative performance, and the case for using it as your primary structural adhesive.

How to Make Casein Glue — The VAKA formula in full: ingredients, ratios, preparation sequence, and troubleshooting. Covers both the powder route and the fresh dairy route, alkali options including potassium and sodium hydroxide, and the role of each ingredient in the final compound.

Shellac: Sealer, Threadlock, and Adhesive

Shellac is produced by the lac insect and has been used in woodworking, instrument making, and boat construction for centuries. In VAKA construction it does three distinct jobs, each requiring a slightly different formulation.

As a sealer and primer, a dilute alcohol shellac solution (1.5–2 pound cut) applied to bare timber seals the grain, improves varnish adhesion, and closes end grain that would otherwise drink topcoats. As a threadlock, a denser solution (2.5 pound cut) applied to the screw thread and driven wet locks fastenings and seals the hole against water ingress — reversibly, with methylated spirits or any high-strength alcohol. As an adhesive, shellac dissolved in ammonia rather than alcohol produces a moderate-strength bonding compound suited to lightweight fittings and small hardware.

One important restriction: alcohol shellac only for use near fastenings. Ammonia reacts with copper, bronze, and brass, causing stress corrosion cracking. Since almost all wooden boat fastenings are silicon bronze, brass, or copper, ammonia shellac belongs nowhere near them.

The post also covers the pound cut system, metric conversion, garnet and blonde grades, dewaxing, shelf life, and the Herreshoff double-planking connection.

Shellac for Boat Building — The complete guide to shellac as sealer, primer, reversible threadlock, and adhesive. Includes the cut/metric reference table and the ammonia activation strength comparison.

Rubberised Marine Glue: Bedding and Seam Compound

The rubberised pitch compound is the hot-applied thermoplastic sealant used 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 in the material hierarchy below casein structurally, but it handles tasks that casein cannot: permanent flexible sealing in locations subject to ongoing hull movement, compression sealing around metal fittings, and seam waterproofing over cotton. The natural rubber fraction gives it elasticity and snap-back under hull flex. The linseed oil keeps it flexible through freeze-thaw cycling — the failure mode of cheaper traditional pitch compounds that use higher shellac proportions.

It is applied hot to pre-warmed timber, which means it penetrates end grain and fills voids around fittings in a way that cold-applied sealants cannot. It is not fast and it is not easy. It rewards patience and preparation.

Traditional Rubberised Marine Glue and Bedding Compound — The full recipe, preparation method, and application notes, including sourcing guidance and honest caveats about where the formulation has and has not been fully tested.

Where Each Material Is Used in a VAKA Build

To make the system concrete, here is where each material appears in a typical VAKA skin on frame construction:

Casein glue: all structural timber-to-timber joints — lashing reinforcement at frame junctions, gunwale-to-frame bonding, thwart knees, any joint that bears direct structural load.

Alcohol shellac (2 pound cut): end grain sealing on all cut timber before varnishing; priming coat on all bare interior timber before Le Tonkinois.

Alcohol shellac (2.5 pound cut): all screw holes and fastenings before driving home — deck fittings, fairleads, cleats, thole pins, any metal fastening into timber.

Ammonia shellac: bonding lightweight interior fittings, fabric trim attachment, small hardware where casein is unnecessary and reversibility is preferred.

Rubberised pitch compound: all through-hull fittings; bedding under any deck hardware that penetrates the skin; seam sealing between stems and the canvas skin

The boat plans specify which material is used at each stage for each design. The full Field Notes knowledge base covers the wider context: hull design, skin treatment, natural varnish, and the ecological reasoning behind building this way.