Shellac Glue, Filler, Sealer and Threadlock

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub

Shellac for Boat Building: Sealer, Threadlock, and the Ammonia Glue Question

Shellac is secreted by the lac insect — Kerria lacca — onto the branches of trees across South and Southeast Asia, scraped off, processed into flakes, and dissolved in alcohol. That is the whole supply chain. No petroleum, no synthetic chemistry, no safety data sheet. The dried film is non-toxic, fully reversible with the solvent it was dissolved in, and has been used in woodworking, instrument making, finishing, and boat construction for centuries. In VAKA construction it does three distinct things: it seals bare timber before varnishing, it locks fastenings and seals screw holes, and — in a separate ammonia-dissolved form — it provides a light-duty adhesive for internal fittings. A fourth use, as a component in the rubberised bedding compound, is covered in the bedding compound post.

It is not a structural adhesive. That job belongs to casein. Understanding what shellac actually is, what it does well, and where its limits are is most of what you need to use it effectively.

What Shellac Is and Where to Start

The London Shellac Research Bureau published a series of technical papers on shellac properties and applications through the 1930s, and they are worth reading if you want the chemistry in detail. The short version: shellac is a complex mixture of resin acids and wax esters, the exact composition varying by lac source, processing method, and grade. The wax content is the key variable for most woodworking applications. Unwaxed shellac — sometimes called dewaxed — has had the natural wax removed, either during processing or by the home procedure described below. Waxed shellac contains the natural wax fraction, which interferes with adhesion of subsequent coatings.

For everything I use shellac for in boat construction, dewaxed is the correct choice. The exception is the rubberised bedding compound, where the waxy grade contributes to the self-sealing behaviour of the finished compound — see that post for the reasoning.

Grades matter less than the dewaxed/waxed distinction, but they're worth knowing. Garnet shellac is the deeper, warmer amber — it comes from a later processing stage and has a richer colour. Blonde and super-blonde are paler and more neutral on light timbers. For most boat construction, garnet or standard orange/amber dewaxed flakes are the working choice. The colour on interior timber is actually rather good.

Dewaxing at home. Dissolve standard flakes in methylated spirits at roughly 1 part flakes to 5 parts solvent by weight — a 2-pound cut or thereabouts. Leave undisturbed for 24 to 48 hours. The wax settles as a pale deposit at the bottom of the container. Decant carefully without disturbing it. The poured-off liquid is dewaxed and ready. Pre-dewaxed flakes from finishing suppliers save this step and are worth the slightly higher cost for regular use.

On the solvent. Methylated spirits is the standard choice in the UK and the cheapest option. Any alcohol above roughly 95% purity works — bioethanol sold for heating fires, isopropyl alcohol, denatured alcohol. What you are avoiding is water content, which causes blushing: the shellac goes cloudy as it dries, harmless for most purposes but poor on a sealing coat. Methylated spirits may leave a faint residue from the denaturing agents, which doesn't matter for boat construction.

Shelf life. Dry flakes keep for years in a sealed container away from moisture. Mixed solution is usable for roughly six months to a year before the resin begins to hydrolyse and the film stops drying hard. Kelly, in his 1921 wood finishing manual, notes the same — old shellac varnish that stays tacky has gone off and should be remade. Test any batch you're unsure about on scrap: fresh shellac dries to a hard, clear film within minutes. Degraded material stays soft.

The Pound Cut System and What Each Concentration Is For

"Pound cut" is the traditional measure of shellac concentration: the number of pounds of dry flakes dissolved in one US gallon of solvent. Converting to metric: one pound cut equals approximately 119 grams per litre. Multiply the cut number by 119 to get grams per litre.

For practical workshop use: a 2-pound cut as 120g of flakes dissolved in 500ml of spirits is a useful working quantity for sealing and priming. Make it up in a small glass jar, label it with the date. I keep this ready at all times during a build — end grain sealing, priming before Le Tonkinois, the odd repair. Having it made up and to hand saves time.

Weigh the flakes rather than measuring by volume. Flake density varies enough by grade and moisture content to make volume measurements unreliable.

As a Sealer and Primer Under Natural Varnish

Bare timber before its first coat of Le Tonkinois will absorb the varnish unevenly — end grain will drink it, flat grain will take it patchily, and the final surface will be irregular. A 1.5 to 2-pound cut shellac applied to the bare wood, allowed to dry fully and then sandpapered lightly before the next coat, seals the grain and creates a more uniform foundation. The sandpapering matters: Kelly notes that some finishers avoid shellac under oil varnish precisely because adhesion between the two can be unreliable, and that the fix is to sand the shellac coat before overcoating. If the shellac goes on without being sandpapered, adhesion between it and the subsequent varnish can fail. Sand it and the problem doesn't arise.

It's also worth knowing that Kelly himself is equivocal about shellac as a sealer — he notes it costs more than standard varnish and "in most cases possesses no advantage over ordinary varnish surfacer; in fact, some finishers prefer the latter." The alternative is to skip the shellac entirely and use thinned Le Tonkinois as the first sealing coat, building up from there. For most timber species this works well. Where I find shellac adds real value is on end grain specifically, which drinks varnish in a way that a diluted first coat doesn't fully address, and on very open-grained timber where the shellac fills the surface before any varnish goes on.

As a Reversible Threadlock for Fastenings

This use matters a great deal in natural construction, and I'll explain why before the method.

A screw driven into timber without any sealing is a water ingress point. The thread contacts end grain and side grain in the hole, and both will wick moisture into the timber if allowed to. In a wooden hull, water that reaches the frame through a fastening hole will promote decay at exactly the point where the structural loads concentrate. Conventional solutions — synthetic thread-locking compounds, polyurethane sealants — work but are permanent, incompatible with natural varnish systems, and inconsistent with the principle that every fastening in this hull should be removable for maintenance and repair without special equipment.

Shellac solved this problem for me.

The method. Apply a 2.5-pound cut shellac solution to the screw thread — brush it on, or work it into the pre-drilled hole with a thin brush — and drive the fastening home while the shellac is still wet. As the alcohol evaporates, the resin hardens in the threads. The fastening is locked and the hole is sealed against water ingress. The whole thing takes about thirty seconds per fastening.

Removal. Work a few drops of methylated spirits into the thread. Wait a minute. The shellac dissolves and the screw extracts without damage to the timber or the fastening. I've used this on cleats and deck fittings that have been in place through several sailing seasons and they've come out cleanly. Nothing toxic, nothing permanent.

One critical restriction: alcohol shellac only for fastenings. Ammonia reacts with copper, bronze, and brass, causing stress corrosion cracking in those metals. Since almost all wooden boat fastenings are silicon bronze, brass, or copper, ammonia-dissolved shellac belongs nowhere near them. This is not a marginal risk — it is a real failure mode that can destroy fastenings invisibly over time. Alcohol shellac only at any fastening point.

If you want to lubricate the screw during driving as well as seal it — which is useful in dense hardwoods — apply tallow or beeswax to the thread first, then the shellac. They are compatible.

The Herreshoff connection. Nathanael Herreshoff, designer of six successive America's Cup defenders, used shellac-saturated silk as the interlayer between the two skins of his double-planked hulls. The silk, saturated with shellac, acted as both sealant and bonding layer — preventing water ingress at the seam and adding structural coherence to the assembly. The precise formulation is not definitively documented, and whether it was alcohol or ammonia shellac is unclear. Given the extended working time needed during planking, plain alcohol shellac as a sealant-adhesive is the more likely candidate. Either way, the principle — shellac as a structural sealing layer in a timber hull — has excellent precedent.

Waterproofing Paper Charts

A practical and completely satisfying application that doesn't fit neatly into the other categories. Navigation charts, tide tables, and log sheets can be waterproofed with a 1-pound cut shellac applied in two thin coats, full drying between them. The shellac penetrates the paper fibres, stiffens the sheet slightly, and renders it substantially water-resistant without obscuring the print or making the surface unwritable with pencil. Keep the coats thin — a heavier application produces a stiffer result that works for charts living permanently in a chart holder but will crack when folded. For notes and log pages that need to survive a wet cockpit, a single thin coat on both sides is sufficient.

Garnet gives the paper a warm amber tint that is perfectly readable and rather pleasing. Super-blonde if you want to preserve the original colour. The whole process takes ten minutes. It is considerably more reversible than lamination and considerably more useful than a waterproof case that gets left below.

Ammonia Shellac: The Adhesive Application

When shellac is dissolved in ammonia rather than alcohol, the chemistry changes in ways that matter. Ammonia is itself a solvent for shellac resin — no alcohol is needed or desirable — and reacts with the resin to form ammonium shellacate, a mobile, tacky solution with genuine adhesive properties beyond those of alcohol shellac. The other significant difference is working time: ammonia evaporates more slowly than alcohol, so the open time is longer, not shorter. Instructions for "ammonia shellac glue" that suggest urgency are wrong.

Mixing. Place dry flakes in a sealed glass jar and cover with household ammonia (10% solution, sold as a cleaning product). Use roughly 3 parts ammonia to 1 part flakes by weight. Seal and leave at room temperature — dissolution takes several hours. Speed it up by placing the sealed jar in a warm water bath at around 50°C for 30 to 60 minutes, shaking occasionally. The result is a dark amber, viscous, pungent solution. Work in a ventilated space. Do not add alcohol.

What it is and isn't good for. The bond strength of ammonia shellac is lower than well-made casein — considerably lower. It is not a structural adhesive for load-bearing frames. The right applications are lightweight internal fittings, attaching small hardware and trim, securing lining fabrics, and consolidating minor repairs where the joint carries no structural load. Its advantages in these contexts are long working time, good tack, reversibility with ammonia solvent, and complete compatibility with every other natural material in the system.

End-grain bonds are weak, as they are with most natural adhesives. Surface-to-surface and lap bonds in shear are where it performs best.

And again: not near fastenings. Ammonia and copper alloys do not mix. This cannot be overstated. Alcohol shellac for fastenings, always.

Ammonia Shellac and Wood Flour: A Grain Filler Worth Testing

One application I've been experimenting with is using ammonia shellac as the liquid component in a wood flour filler paste. The principle is straightforward: mix fine wood flour — from the same timber species if possible, or a close match — into ammonia shellac solution at a 3-pound cut until you get a thick, workable paste. Apply it to damaged grain, open knots, small voids, or surface checks, press it in firmly, and allow the ammonia to evaporate fully before sanding back. The result is a filler that is chemically related to the wood surface, compatible with the shellac primer coats that follow, and biodegradable.

The practical advantage over conventional fillers is compatibility. Proprietary wood fillers are typically acrylic or polyester-based and sit awkwardly in a natural material system — they may accept varnish but they aren't part of the same material family, and they won't compost with the hull at end of life. A shellac-and-wood-flour paste is entirely of a piece with everything else in the finishing schedule.

The ammonia shellac is the right base for this rather than alcohol shellac because the longer open time lets you work the paste into voids and tool it to shape before it begins to set. Alcohol shellac skins too quickly for anything more than the smallest repairs. The paste stiffens as the ammonia evaporates, shrinks slightly as it cures, and sands cleanly when fully dry. On the shrinkage: overfill slightly and sand back rather than trying to fill flush. Two applications may be needed on deeper voids.

I'm still accumulating evidence on long-term durability under varnish — how the filled areas behave through wetting and drying cycles, whether the filler moves differently from the surrounding timber. Early results look good but this is work in progress.

How It Fits With the Rest of the System

Shellac, casein, and the rubberised bedding compound form a complete natural adhesives and sealants system for skin on frame construction. Each has a defined role. Casein takes the structural loads. Shellac seals, primes, locks fastenings, and handles light bonding. The rubberised compound beds hardware and pays seams. None of them requires specialist disposal. All of them are reversible. All of them biodegrade.

The adhesives hub covers how they fit together and where each is specified in a VAKA build. The boat plans note the specific application at each joint and fitting.

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

London Shellac Research Bureau (1936). Technical Papers. London. 

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

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

Forest Products Laboratory, U.S. Department of Agriculture (1961). Casein Glues: Their Manufacture, Preparation, and Application. Report No. 280. Available via Oregon State University library: