VAKA's Sustainable & Traditional Rubberized Marine Glue & Bedding Compound

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub 

Subject: Traditional Rubberised Marine Glue and Bedding Compound

Field Entry: April 15, 2026

There is a category of traditional marine sealant that sits between a structural adhesive and a bedding compound — neither fully rigid nor permanently flexible, waterproof, self-healing under moderate compression, and compatible with timber movement in a way that modern synthetic alternatives often are not. Rubberised marine glue is what shipwrights used before polysulphide and polyurethane sealants existed, and in the context of natural material boat construction it remains the most coherent choice for through-hull fittings, seam sealing, and deck hardware bedding.

This post covers my in-progress formula, the preparation method, and the application notes for using it in skin on frame construction. It is part of the guide to natural marine adhesives and sealants that covers the complete bonding system used in VAKA builds — casein glue for structural joints, shellac for sealing and threadlocking, and this compound for the applications where you need waterproof, permanently flexible bedding that tolerates hull flex without cracking out.

What This Compound Is and What It Does

Traditional rubberised marine glue is a hot-applied thermoplastic sealant and bedding compound made from pine-derived pitch, natural rubber, shellac, tar, and linseed oil. It was widely used in wooden boat construction through the nineteenth and early twentieth centuries for paying seams over cotton caulking, bedding through-hull fittings, and sealing deck hardware. It fell out of favour when synthetic polysulphide sealants became available in the 1960s — not because the synthetic alternatives were better in all respects, but because they were easier to apply cold and faster to cure.

The synthetic alternatives have their own problems in a natural construction context. Polysulphide and polyurethane sealants are petroleum-derived, are not biodegradable, cannot be reversed or cleaned up with natural solvents, and are often incompatible with natural varnish systems. For a hull where every other material — timber, canvas, casein glue, shellac, linseed oil — is natural and reversible, a synthetic bedding compound is an odd compromise.

The rubberised compound has several properties that make it well-suited to skin on frame work specifically. It remains flexible at low temperatures, unlike hard pitch compounds that become brittle and crack in cold weather. It is self-healing under compression — when a fitting is tightened down, the compound flows slightly to fill voids. It is fully compatible with linseed oil treatments, pine tar, and shellac. And it is applied hot, which means it penetrates into end grain and around fittings in a way that cold-applied sealants cannot.

It is not, however, a structural adhesive. It does not replace casein for structural joints, and it does not replace cotton caulking in seam applications — it pays over the cotton, not instead of it. Understand what it is for and it is an excellent material. Use it outside that envelope and you will be disappointed.

The VAKA Formula

This formulation has been developed to address the main failure mode of inferior traditional marine glues: cold-weather brittleness. Older recipes with high shellac proportions tend to become hard and crack under freeze-thaw cycling. The inclusion of stand oil and a reduced shellac fraction keeps the cured compound flexible across the temperature range that UK coastal sailing involves.

A few sourcing notes. Brewer's pitch should be pure pine-derived — not the paraffin-extended versions sometimes sold as wood pitch, which introduce a petroleum component and reduce flexibility. Stockholm tar is widely available from agricultural and equestrian suppliers; it is the same product used for hoof treatment. Unvulcanised natural crepe rubber is sold in sheet form for eraser manufacture and is available from craft suppliers. Do not substitute vulcanised rubber — the chemistry is different and it will not dissolve...trust me on this!

The shellac in this formula is intentionally waxy — the standard, un-dewaxed grade. The wax contributes to the compound's self-sealing behaviour and its tack against timber. This is one of the few applications in VAKA construction where dewaxed shellac is not the better choice.

Preparation

Step 1 — Rubber dissolution (48 hours to 2 weeks ahead)

shred the crepe rubber on a cheese grater— and place in a sealed glass jar. Cover just to depth with gum turpentine. Seal and leave at room temperature. The rubber will swell and soften over 48 hours or longer into a dense, stringy gel. It will not dissolve completely; you want a thick, coherent gel rather than a solution. If the rubber is still firm after 24 hours, leave it longer. There is no advantage to rushing this stage.

Gum turpentine here is purely a dissolution medium for the rubber. It is not a permanent ingredient — most of it will evaporate during and after the melt stage.

Step 2 — The melt

Using a double boiler or slow cooker melt the brewer's pitch gently. . Do not use direct flame. Pitch is flammable, and gum turpentine vapour from your rubber gel is also present in the workshop at this stage. Work in a ventilated space and keep a lid available to smother the inner pot if needed.

Once the pitch is melted and fluid, add the shellac flakes a little at a time, stirring until fully dissolved. The mixture will darken and become more viscous. Add the shellac slowly and stir constantly to avoid lumps.

Step 3 — Oil and tar

Once the pitch and shellac are smooth and fully combined, stir in the Stockholm tar, then the stand oil. Add each separately and stir well between additions. The linseed stand oil is the key ingredient for cold-weather performance: it prevents the shellac from dominating the cure character and gives the finished compound long-term flexibility rather than the brittleness that high-shellac formulas develop in winter.

Step 4 — Rubber integration

Remove the pot from the heat briefly and allow the temperature to drop slightly — below vigorous bubbling. Then slowly work in the rubber gel, a spoonful at a time, stirring continuously. The rubber will resist at first and then begin to incorporate into the melt as it warms. Take your time. If the mixture starts to bubble aggressively, remove from heat and wait. You want thorough integration without driving off too much of the turpentine too quickly.

The finished melt should be smooth, dark, viscous, and slow-moving when poured. If it is too stiff, a small additional amount of Stockholm tar will loosen it. If it is too fluid, return briefly to gentle heat to drive off more solvent.

Step 5 — Cast and store

Pour into greased tins or silicone moulds to form pucks or blocks. A thin smear of tallow in the mould prevents sticking. Allow to cool fully before removing — the compound will be firm and slightly tacky at room temperature. Store in a sealed container away from direct sunlight, which will accelerate oxidation of the linseed oil fraction over time.

For use: reheat in a double boiler or slow cooker. The compound remelts cleanly and repeatedly without degradation.

Application

Prepare the timber first. Pre-heat the wood surface with a heat gun before applying. Cold, damp timber will cause the compound to skin on contact rather than penetrate. The objective is a surface that is warm and bone dry — the hot compound should flow into the grain rather than congealing on the surface.

For through-hull fittings and screw holes. Pour or work in hot with a small stick. Allow the compound to overflow slightly around the fitting before tightening down. The overflow will be compressed into the joint as the fastening is driven home, filling any voids. Trim the cooled excess flush with a sharp chisel. For screw holes, the compound seals the thread and the end grain of the timber simultaneously — use it in combination with shellac threadlocking where additional mechanical locking is required.

For seam sealing in carvel planked boats. Cotton caulking goes in first — the rubberised compound pays over the cotton, not as a structural replacement for it. The cotton does the structural work of bridging the seam under movement; the compound waterproofs and seals over the top. Apply hot with a seam iron or pour carefully into the seam. Work in short sections; the compound skins quickly in cold weather.

Cleanup. Gum turpentine while the compound is still warm. Gentle heat to reflow any overspill before cleaning. Cured compound that has been sitting for days is harder to shift — warm it first.

Honest Caveats

This is still an experimental formulation. The recipe has been developed and tested through use, but it has not been through systematic freeze-thaw and immersion testing that would produce a definitive performance specification.

The compound skins within minutes of application but does not fully cure for several days. Do not immerse work within 48 hours of application if you can avoid it.

Where it works well — bedding deck hardware on a skin on frame hull, sealing screw holes against water ingress, paying seams over cotton caulking — it is a genuinely good material: natural, reversible, compatible with every other element of the VAKA system, and available to anyone with a double boiler and an afternoon.

The boat plans note where this compound is used at each stage of the build. The full Field Notes knowledge base covers the wider material and construction context.