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

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub 

How to Make Traditional Rubberised Marine Glue for Seam Paying and Through-Hull Bedding — A Pitch, Shellac, Linseed and Rubber Formula

There is a class 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 built for the movement that happens in wooden hulls. It is what shipwrights used to pay seams over cotton caulking, bed through-hull fittings, and secure deck hardware before polysulphide and polyurethane arrived in the 1960s and pushed hot-applied marine glue out of the working boatyard.

It took me several batches to get a formula that works through a British winter, and every failure failed the same way. The casein posts and the shellac post cover the adhesive and sealing work in a VAKA build. This is the third material in that natural system, rubberised pitch compound. The hierarchy is simple. Casein handles structural loads, shellac seals and locks fastenings, rubberised compound beds hardware and pays seams. None of those jobs could reasonably be done by the other two.

What it is, and why it has to go on hot

Chapelle, in his 1941 boatbuilding manual, describes the traditional paying procedure in plain terms. Heat the compound until fluid, pour it into the seam with a lipped ladle held two to three inches above the deck to prevent air bubbles, fill in two passes to half depth each time, scrape flush when cold. He notes some compounds melt directly in an iron ladle and others need a steam bath. That’s the working description of the material. Thermoplastic, hot-applied, poured into place.

The heat is not a complication to work around. It is the reason the material works. A compound poured hot penetrates end grain and fills the awkward geometry around through-hull fittings in a way cold-applied sealants cannot. It also drives off surface moisture at the moment of application. Cold, damp timber makes the compound skin on contact rather than penetrate, so pre-warming the surface with a heat gun is not optional. The same Chapelle volume confirms what I’d found in scattered references about double-planking: between the two skins there is often silk or muslin, laid on the inner skin with marine glue, paint, varnish, or occasionally aircraft dope. That is the Herreshoff technique, shellac-saturated silk as an interlayer sealant, appearing in mainstream 1941 boatbuilding instruction as established practice. The same family of hot-applied natural compounds, across all of it.

The formula

The starting point is Henley’s Twentieth Century Formulas, Recipes and Processes, first published in 1896 and updated through several editions into the early twentieth century. Henley’s gives several marine glue recipes, and the core of them combines pitch, shellac, and rubber as the three components that matter. That basic combination is sound and well-tested. What I’ve changed is the balance. The Henley’s formulas run to higher shellac proportions, which give a harder compound that does well in warm climates and goes brittle in a British winter. Getting to something that survives a freeze-thaw cycle took several batches, and each failure taught the same lesson: 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. It keeps the cured compound flexible and stops the shellac dominating. The formula I use now, by parts weight:

A few sourcing notes that matter. Brewer’s pitch should be pure pine-derived, not the paraffin-extended versions sometimes sold as wood pitch, which add a petroleum component and reduce flexibility. Stockholm tar comes from agricultural and equestrian suppliers, the same product used for hoof treatment, and it has been used at sea for centuries. The unvulcanised natural crepe rubber is sold in sheet form for eraser manufacture, stocked by craft suppliers. Do not substitute vulcanised rubber. The chemistry is different and it won’t integrate into the melt. The shellac here is the waxy, un-dewaxed grade, because 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. And the boiled linseed oil is the ingredient that earns its place in cold weather, keeping the compound flexible through freeze-thaw and preventing the shellac from setting the cure character.

Preparation

Start the rubber a day or two ahead. Dice the crepe rubber into small cubes, 5 to 10mm, put them in a sealed glass jar, and cover just to depth with gum turpentine. Seal it and leave it at room temperature. Over 24 to 48 hours the rubber swells into a dense, stringy gel. You want a thick coherent gel, not a solution. The turpentine is purely a dissolution medium and most of it evaporates during the melt. Do not use white spirit or mineral turpentine, which leave a petroleum residue that affects the cure.

Melt the pitch and shellac next, in a double boiler, an inner pot in an outer water bath. No direct flame. Pitch is flammable and there is gum turpentine vapour from the rubber gel in the workshop, so keep a lid to hand. Melt the brewer’s pitch gently, then add the shellac flakes a little at a time, stirring until fully dissolved. The mixture darkens and thickens. Once it’s smooth, stir in the Stockholm tar, then the boiled linseed oil, adding each separately and stirring well between.

Then the rubber. Take the pot off the heat and let the temperature drop below vigorous bubbling. Work in the rubber gel a spoonful at a time, stirring continuously. It resists at first and then incorporates as it warms. Take your time. If the mixture starts bubbling aggressively, take it off the heat and wait. The finished melt should be smooth, dark, viscous, and slow-moving when poured. Too stiff, add a little more Stockholm tar. Too fluid, return it to gentle heat briefly to drive off more solvent.

Cast it into greased tins or silicone moulds, a smear of tallow to stop it sticking, and let it cool fully. Store the pucks in a sealed container out of direct sunlight, which oxidises the linseed oil fraction over time. To remelt, double boiler again. 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 makes the compound skin on contact instead of penetrating. This step is not optional.

For through-hull fittings and screw holes, work the hot compound in with a thin stick, let it overflow slightly, then tighten the fitting down. The overflow compresses into any remaining voids as the fastening drives home. Trim the cooled excess flush with a sharp chisel. This works alongside shellac threadlocking, shellac on the screw thread and 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, and the compound waterproofs and seals over the top. Pour or apply it hot, work in short sections, and let each section cool before moving on. In cold weather the compound skins quickly, so work fast enough to stay ahead of it. For cleanup, gum turpentine while still warm, gentle heat to reflow any overspill before wiping, and warm any cured compound that has been sitting for days before you try to lift it.

Honest caveats

This formulation has been developed through use but hasn’t been through the systematic freeze-thaw and immersion testing that would give a definitive performance spec. The failure mode of higher-shellac compounds, cold-weather cracking, is well documented in the traditional sources. The reduced shellac fraction and the linseed oil are specifically meant to prevent it, but test a small puck through a freeze-thaw cycle before you commit the compound to a seam that matters. Curing time matters too. It skins quickly but doesn’t fully cure for several days, so don’t launch work within 48 hours of application if you can avoid it.

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: