Shellac Glue, Sealer and Threadlock
Collection: Field Notes - Regenerative Materials
Series: Natural Marine Adhesives & Sealants Hub
Subject: Shellac Glue for Boat Building
Shellac for Boat Building: Sealer, Threadlock, and Natural Adhesive
The lac insect secretes this resin onto tree branches across South and Southeast Asia. Harvested, processed, and dissolved in alcohol, it becomes one of the most versatile natural materials a small craft builder can have on the shelf. It biodegrades. It is non-toxic once cured. Any application is fully reversible with the solvent it was mixed with — which, as anyone who has tried to undo a synthetic adhesive with a heat gun and a chisel will appreciate, is not a minor advantage.
I use it in four distinct ways in VAKA construction: as a sealer and primer under natural varnish, as a reversible threadlock and screw-hole sealant, as a bonding glue when dissolved in ammonia, and as a component in the traditional rubberised marine bedding compound. The threadlocking application is alcohol shellac only — ammonia reacts with copper, bronze, and brass, which rules it out for use with the screws, rivets, and fastenings that hold a wooden boat together. It connects to the wider natural marine adhesives guide and the casein glue guides that cover the primary structural adhesive in VAKA construction.
Shellac Flakes and Garnet Grade: Buying, Dewaxing, and Shelf Life
The material is sold in several forms, and the differences matter for practical use. The most versatile starting point is dewaxed shellac flakes — dry, amber-coloured wafers that dissolve in alcohol to produce a working solution. Standard flake shellac that has not been dewaxed contains natural wax, which interferes with the adhesion of subsequent coatings and reduces bond strength in adhesive applications. For all the uses described in this guide, use the dewaxed form.
The solvent
Methylated spirits is the obvious and often cheapest choice in the UK. But any alcohol above roughly 95% purity will work — bioethanol (sold for bioethanol fires and readily available online) is a good alternative, as is isopropyl alcohol (IPA). Denatured alcohol and pure ethanol also work. What you are looking for is high purity and low water content; water in the solvent causes the shellac to blush (go cloudy) as it dries, which is harmless for most applications but looks poor on a sealing coat. Methylated spirits may leave a slight residue from the denaturing additives, which is generally not a problem for boat construction purposes.
Garnet shellac is a deeper, warmer amber variety produced from a different processing stage. Perfectly functional for sealing and bonding, and it produces a lovely rich colour on timber — which matters on interior surfaces. Blonde or super-blonde grades are lighter and useful where you want to minimise colour shift on pale wood. For most boat construction, garnet or standard amber dewaxed flakes are the practical choice.
If dewaxed flakes are not available locally, standard material (which is also cheaper) can be dewaxed at home. Dissolve the flakes in methylated spirits at roughly 1 part flakes to 5 parts solvent by weight — approximately a 2-pound cut which is a general purpose strength (see below). Leave to stand undisturbed for 24 to 48 hours. The wax will settle to the bottom as a pale deposit. Pour off the clear solution carefully, leaving the wax behind. Don't shake it before decanting. The poured-off liquid is dewaxed and ready.
Pre-dewaxed flakes are available from specialist finishing suppliers and save this step. One traditional natural light amber stick shellac — the kind used by woodturners — is also dewaxed and can be applied directly with a heated iron. For boat construction, the flake form dissolved in spirits is more versatile.
Shelf life and storage
Dry flakes in a sealed container away from moisture keep for years. Mixed solution lasts roughly six months to a year before the resin begins to break down and lose potency — the ester linkages hydrolyse slowly, and the result is a solution that stays tacky rather than drying hard. Mix in small quantities as needed. To test a batch: apply a little to scrap timber. Fresh material dries hard and clear within minutes. Suspect material stays soft and should be discarded.A practical working concentration for most sealing and priming uses is 1.5 to 2-pound cut — dilute enough to penetrate well into the timber surface. For threadlocking, use a denser 2.5-pound cut; you want body in the thread. For ammonia glue, shellac is dissolved without alcohol as described in the final section. I keep a working bottle of 2.5-pound cut made up at all times; it saves a lot of time on a build day and can be diluted further as needed.
Understanding the pound cut — and converting to metric
"Pound cut" is a traditional measure: the number of pounds of dry flakes dissolved in one US gallon of solvent. Converting to metric is straightforward. One pound cut equals approximately 119 grams of flakes per litre of solvent.
| Cut | Grams per litre | Approx ratio by weight | Primary uses |
|---|---|---|---|
| 1 lb | ~120 g/l | 1:8 | Wash coat, grain raising, thin sealer |
| 1.5 lb | ~180 g/l | 1:5.5 | First sealer coat on bare timber, end grain |
| 2 lb | ~240 g/l | 1:4 | General sealing, priming under varnish, dewaxing base |
| 2.5 lb | ~300 g/l | 1:3.5 | Threadlocking, screw-hole sealing |
| 3 lb | ~360 g/l | 1:3 | Thicker finishing coats, base for ammonia glue |
For practical workshop quantities, a 2-pound cut as 120 g of flakes in 500 ml of spirits is a convenient working amount. Weigh the flakes rather than measuring by volume — flake density varies enough to make measuring by volume too hit and miss.
As a Sealer, Primer, and Reversible Threadlock
The sealing properties of this resin derive from its film-forming chemistry: as the alcohol evaporates, a thin, hard, non-porous film deposits onto the timber surface, seals the grain, and provides a compatible primer for natural varnish systems such as Le Tonkinois. One or two thin coats on bare timber, allowed to dry fully between coats, dramatically improve the evenness of subsequent varnish absorption. End grain — always the problem area — seals particularly well; without a sealing coat, end grain drinks varnish and never properly closes. Apply with a brush or lint-free pads, allow to harden, and then varnish. The dried film sands lightly and creates a good foundation for oil varnish.
As a reversible threadlock and screw-hole sealant, alcohol shellac is hard to beat. The approach: apply a 2.5-pound cut shellac solution to the pilot hole with a small syringe or work it into the hole with a thin brush — and drive the screw home while the solution is still wet. As the alcohol evaporates (which it will through the wood, don't worry), the resin hardens in the threads, locking the fastening and sealing the hole against water ingress simultaneously.
Use alcohol shellac only for fastenings — never ammonia shellac. Ammonia reacts with copper, bronze, and brass, and can cause stress corrosion cracking in those metals. Since almost all wooden boat fastenings — screws, rivets, keel bolts, cleats — are silicon bronze, brass, or copper, using ammonia anywhere near them is asking for trouble. Alcohol shellac has no such issue and is the correct choice here. If you want to lubricate the screw during driving, apply beeswax on the thread — the two are compatible and serve different purposes.
This is particularly valuable at deck fittings, fairleads, and any hardware penetrating the hull skin or deck fabric — points where a damp thread would otherwise wick water down to the timber frame below and quietly start rotting it. A properly sealed screw hole stays dry for years and still comes out cleanly when needed.
The release mechanism is the same as the application: a few drops of methylated spirits (or any high-proof alcohol) worked into the thread. Leave it for a minute, and the screw extracts without damage to the timber or the fastening. I've used this on fittings that have been in place for several seasons and they've come out without difficulty. Nothing toxic, nothing permanent, nothing that requires a specialist to undo.
The Herreshoff connection
Nathanael Herreshoff — designer of six successive America's Cup defenders — used shellac on the silk interlayer between the two skins of his double-planked hulls. The silk, saturated with shellac, acted as both sealant and bonding layer between planks, preventing water ingress at the seam and adding structural coherence to the assembly. The precise formulation isn't definitively documented — perhaps ammonia shellac was used, given the long working time needed during planking or perhaps alcohol shellac was used with very fast skilled builders! Either way, the principle is sound and the precedent is excellent. It was also used to seal joints between wood that was required to be dismantled.
Waterproofing paper charts.
A practical and somewhat satisfying application: paper charts, tide tables, and navigation notes can be waterproofed with a 1-pound cut alcohol shellac applied in two thin coats, allowing full drying between them. The shellac penetrates the paper fibres, stiffens the sheet slightly, and renders it substantially resistant to water without obscuring the print or making the surface unwritable with a pencil. The paper remains flexible enough to fold without cracking if the coats are kept thin. A heavier coat produces a stiffer, more waterproof result that works well for charts that will live permanently on a chart table rather than being folded or rolled. For log pages, pilot notes and anything that needs to occasionally survive a wet cockpit, a single thin coat on both sides is sufficient. Garnet shellac gives the paper a warm amber tint that is perfectly readable in daylight and rather pleasing; if you want to preserve the original colour, use super-blonde. The process takes ten minutes and costs almost nothing. It is considerably more reversible than varnish or lamination, and considerably better for the environment than a plastic folder that will lie longer than you. And your kids. And their kids. Just sayin
Ammonia Activation: Strength, Properties, and the Natural Glue Application
When shellac is dissolved in ammonia rather than alcohol, the chemistry and working properties change significantly. Ammonia is itself a solvent for shellac resin — no alcohol is needed or desirable. The ammonia molecule reacts with the shellac resin to form ammonium shellacate, a mobile, tacky solution with genuine adhesive properties beyond those of alcohol shellac. In its standard alcohol-dissolved form, shellac is a purely evaporative "setting" adhesive — the bond forms as the alcohol evaporates quickly, leaving a hard resin deposit. The ammonia-dissolved version behaves differently: ammonia evaporates more slowly than alcohol, giving a longer open time and allowing the adhesive to develop better tack before it begins to cure. I've seen instructions for "ammonia shellac glue" sometimes suggest urgency — this is misplaced. You have more time with the ammonia version, not less.
Mixing ammonia shellac. Place dry shellac flakes in a sealed glass jar and cover with household ammonia (10% solution, the kind sold as a cleaning product). Use roughly 3 parts ammonia by weight to 1 part flakes. Seal the jar and leave at room temperature — dissolution takes several hours. To speed things up, place the sealed jar in a warm water bath at around 50°C for 30–60 minutes, shaking occasionally. The result is a dark amber, viscous, pungent solution. Work in a ventilated space. Do not add alcohol — the two solvents serve different purposes and mixing them simply dilutes the ammonia activation without any benefit. And do not use this anywhere near copper, bronze, or brass fastenings — see the threadlocking section above.
Strength, flexibility, and honest uncertainty. Ammonia shellac produces a different glue line than alcohol shellac — the reaction chemistry is different and the cured resin is not identical. Whether the bond is meaningfully more flexible is genuinely uncertain; good comparative mechanical data for ammonia shellac as a structural adhesive is sparse, and the flexibility claim that circulates in various woodworking sources may be overstated. What is clearer is that bond strength sits well below casein, and the application range is correspondingly more modest.
The following table gives indicative figures. <u>The ammonia shellac values are estimates derived from related literature on shellac resin properties and should be treated as approximate; they have not been verified against bonded timber specimens.</u>
| Adhesive | Tensile strength (approx) | Flexural strength (approx) | Elongation to failure | Notes |
|---|---|---|---|---|
| Standard Lime-casein | 8–15 MPa | 10–20 MPa | Low (<2%) | Thermoset, brittle failure — wood fails before the glue line when well-made. Tannic acid improves elongation further. |
| PVA (D3/D4) | 8–12 MPa | 10–15 MPa | Moderate (5–10%) | Thermoplastic, creeps under sustained load, softens with heat |
| Ammonia shellac | 2–6 MPa | 4–8 MPa | Low–moderate (2–5%) | Estimated; data sparse; suitable for light bonding, not structural |
Ammonia shellac is not a structural adhesive for load-bearing frames. Its appropriate applications are lightweight internal fittings, small hardware, securing lining or fabric, and consolidating minor repairs where the joint carries no significant load. The advantage over casein in these contexts is reversibility with ammonia solvent, reasonable working time, and complete compatibility with every other natural material in the system.
Joint design
End-grain bonds are weak, as they are with most natural adhesives. This works best in lap and surface-to-surface bonds where the glue line is in shear. Don't design joints that rely on tensile or peel resistance.
In a natural construction system, shellac fits into a coherent material hierarchy alongside casein for structural joints, the rubberised bedding compound for through-hull fittings and deck hardware, and linseed oil and pine tar for wood preservation. Each material has a defined role and a defined limit. A builder who understands all three has a complete natural adhesives system covering every bonding and sealing task a skin on frame sailing vessel presents — from the first frame joint to the last deck fitting. The full picture is in the natural marine adhesives guide. The boat plans specify which compound is used at each stage of the build, and the Field Notes covers the broader context of natural material construction for small craft.
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