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

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub 

Traditional Rubberised Marine Glue: Seam Paying, Through-Hull Bedding, and a Formula That Took Several Attempts

There is a category of traditional marine sealant that sits between a structural adhesive and a flexible bedding compound — neither rigid nor permanently soft, waterproof, self-healing under compression, and specifically suited to the movement that happens in wooden hulls. It is the material shipwrights used for paying seams over cotton caulking, bedding through-hull fittings, and securing deck hardware before polysulphide and polyurethane sealants arrived in the 1960s. After those synthetics became available, hot-applied marine glue largely disappeared from the working boatyard.

The casein glue posts and shellac post cover the adhesive and sealing work in VAKA construction. This post covers the third material in the natural system: rubberised pitch compound. Where it sits in the hierarchy is simple — casein handles structural loads, shellac seals and locks fastenings, rubberised compound beds hardware and pays seams. None of these jobs could reasonably be done by the other two.

What It Is and Why Hot Application Matters

Chapelle, in his 1941 boatbuilding manual, describes the traditional marine glue paying procedure in practical terms: heat the compound until fluid, pour into the seam with a lipped ladle held two to three inches above the deck surface to prevent air bubbles, fill in two passes to half depth each time, scrape flush when cold. He notes that some compounds can be melted directly in an iron ladle; others require a steam bath. This is the working description of the material — thermoplastic, hot-applied, poured into place.

The hot application is not a complication to be worked around. It is why the material works. A compound poured hot penetrates end grain and fills the irregular geometry around through-hull fittings in a way that cold-applied sealants cannot. The heat also ensures the timber surface is thoroughly dry at the moment of application — if the wood is cold and damp, the compound skins on contact rather than penetrating. Pre-warming the surface with a heat gun before applying is not optional.

The same Chapelle volume also confirms what I'd found in scattered references about double-planking practice: between the two skins there is often silk or muslin, laid on the inner skin with marine glue, paint, varnish, or occasionally aircraft dope. This is the Herreshoff technique — shellac-saturated silk as an interlayer sealant — appearing in mainstream 1941 boatbuilding instruction as established practice. The material being used across these applications is the same family of hot-applied natural compounds.

The Formula

The starting point is Henley's Twentieth Century Formulas, Recipes and Processes — a compendium first published in 1896 and updated through multiple editions into the early twentieth century. Henley's gives several marine glue recipes, the core of which combines pitch, shellac, and rubber as the three essential components. That basic combination is sound and historically well-tested. What I've modified is the balance: the Henley's formulas tend toward higher shellac proportions, which produce a harder compound that performs well in warm climates but becomes brittle in a British winter. Getting to a formula that works through a freeze-thaw cycle took several batches and the failure mode of each one was instructive — always cold-weather cracking, always along the seam, the compound lifting in clean pieces off the cotton caulking beneath.

The addition that makes the difference is boiled linseed oil, which keeps the cured compound flexible and prevents the shellac from dominating. The formula I use now is by parts weight:

A few sourcing notes that matter. Brewer's pitch should be pure pine-derived — not paraffin-extended versions sometimes sold as wood pitch, which introduce a petroleum component and reduce flexibility. Stockholm tar is available from agricultural and equestrian suppliers; it is the same product used for hoof treatment and has been used in maritime applications for centuries. The unvulcanised natural crepe rubber is sold in sheet form for eraser manufacture — craft suppliers stock it. Do not substitute vulcanised rubber; the chemistry is different and it will not integrate into the melt.

The shellac here is the waxy, un-dewaxed grade — the natural wax contributes to the compound's self-sealing behaviour under compression. This is one of the few places in VAKA construction where waxed shellac is the right choice. The boiled linseed oil is the key ingredient for cold-weather performance; it is what keeps the compound flexible through freeze-thaw cycling and prevents the shellac from dominating the cure character.

Preparation

Step 1 — Dissolve the rubber (24–48 hours ahead). Dice the crepe rubber into small cubes, about 5–10mm. Place in a sealed glass jar and cover just to depth with gum turpentine. Seal and leave at room temperature. The rubber swells over 24–48 hours into a dense, stringy gel. You want a thick coherent gel, not a solution. The turpentine here is purely a dissolution medium — most of it will evaporate during the melt stage. Do not substitute white spirit or mineral turpentine; they leave a petroleum residue that affects the cure character.

Step 2 — Melt the pitch and shellac. Use a double boiler — an inner pot sitting in an outer water bath. Do not use direct flame. Pitch is flammable, and gum turpentine vapour from the rubber gel is also present in the workshop. Keep a lid available. Melt the brewer's pitch gently, then add the shellac flakes a little at a time, stirring until fully dissolved. The mixture will darken and thicken.

Step 3 — Add oil and tar. Once the pitch and shellac are smooth, stir in the Stockholm tar, then the boiled linseed oil. Add each separately and stir well between additions.

Step 4 — Integrate the rubber. Remove from heat and allow the temperature to drop below vigorous bubbling. Slowly work in the rubber gel a spoonful at a time, stirring continuously. The rubber will resist at first and then incorporate as it warms. Take your time. If the mixture starts bubbling aggressively, remove from heat and wait.

The finished melt should be smooth, dark, viscous, and slow-moving when poured. Too stiff: add a small additional amount of Stockholm tar. Too fluid: return to gentle heat briefly to drive off more solvent.

Step 5 — Cast and store. Pour into greased tins or silicone moulds. A smear of tallow prevents sticking. Allow to cool fully. Store in a sealed container away from direct sunlight, which oxidises the linseed oil fraction over time. To remelt: double boiler, same as before. The compound remelts cleanly and repeatedly.

Application

Prepare the timber first. Pre-warm the surface with a heat gun until it is bone dry and warm to the touch. Cold, damp timber causes the compound to skin on contact rather than penetrate. This step is not optional.

For through-hull fittings and screw holes. Work the hot compound in with a thin stick, allow it to overflow slightly, then tighten the fitting down. The overflow compresses into any remaining voids as the fastening is driven home. Trim the cooled excess flush with a sharp chisel. This can be used alongside shellac threadlocking — shellac on the screw thread, rubberised compound bedding the fitting's bearing surface.

For seam paying. Cotton caulking goes in first. The compound pays over the cotton — it does not replace it. The cotton does the structural work of bridging the seam under movement; the compound waterproofs and seals over the top. Pour or apply hot, work in short sections, allow each section to cool before moving on. In cold weather the compound skins quickly; work fast enough to stay ahead of it.

Cleanup. Gum turpentine while still warm. Gentle heat to reflow any overspill before wiping. Cured compound that has been sitting for days: warm it first.

Honest Caveats

This formulation has been developed through use but has not been through systematic freeze-thaw and immersion testing that would produce a definitive performance specification. The failure mode of compounds with higher shellac proportions — cold-weather cracking — is well documented in traditional boatbuilding sources. The reduced shellac fraction and the linseed oil addition are specifically intended to prevent this, but you should test a small puck through a freeze-thaw cycle before committing the compound to a seam that matters. The curing time before immersion matters too: the compound skins quickly but does not fully cure for several days. Don't launch work within 48 hours of application if you can avoid it.

At VAKA I design and build boats that don't destroy the environment. Find the plans as they are finalised at VAKA Plans and the full field notes here. If you are looking for a launching spot, the Hithe Finder is a community register of slipways, hards, and beaches for small boats.

VAKA. Traditional craft and natural materials. Nottingham. 2026.

References

Chapelle, H.I. (1941). Boatbuilding: A Complete Handbook of Wooden Boat Construction. W.W. Norton, New York

Henley, W.T. (ed.) (1914). Henley's Twentieth Century Formulas, Recipes and Processes. Available via Project Gutenberg

Gardiner, R.P. (1920). Examination of Pitch as a Possible Source of Resins for Varnish Making. Bachelor of Science thesis, University of Illinois.

Kelly, A. Ashmun (1921). The Expert Wood Finisher. Available via Woodworkers UK: