Shellac Glue, Filler, Sealer and Threadlock

Collection: Field Notes - Regenerative Materials

Series: Natural Marine Adhesives & Sealants Hub

Shellac for Boat Building: How to Use Shellac as a Sealer, Reversible Threadlock, Wood Filler, and Light-Duty Glue

Shellac comes from an insect and dissolves in alcohol, and on a boat it does at least four jobs that have nothing to do with the furniture finish most people file it under. It seals bare timber, locks and waterproofs fastenings, fills open grain, and in a separate ammonia-dissolved form bonds light internal fittings. None of those is structural gluing. That job belongs to casein.

I had filed shellac under “traditional wood finish” and left it there for years longer than the material deserved. What pulled it back out was a fastening problem I’ll come to. The whole supply chain is one organism and one solvent. The lac insect, Kerria lacca, secretes resin onto the branches of trees across South and Southeast Asia. The resin is scraped off, processed into flakes, dissolved in alcohol. 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, and boat construction for centuries.

What the material actually is, and where to start

Shellac is a complex mixture of resin acids and wax esters. The exact composition varies by lac source, processing method, and grade. For most of what I do with it, the variable that matters is the wax content. Unwaxed shellac, often called dewaxed, has had the natural wax removed, either during processing or by the home procedure below. Waxed shellac still carries the natural wax fraction, which sits at the film surface and interferes with the adhesion of anything coated over the top.

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

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

Dewaxing at home is straightforward. Dissolve standard flakes in methylated spirits at roughly one part flakes to five parts solvent by weight, a two-pound cut or thereabouts. Leave it undisturbed for 24 to 48 hours. The wax settles as a pale deposit at the bottom of the jar. Decant carefully without stirring it back up. The poured-off liquid is dewaxed and ready. Pre-dewaxed flakes from finishing suppliers save the step and are worth the small premium if you use shellac regularly.

On the solvent. Methylated spirits is the standard UK choice and the cheapest. Any alcohol above roughly 95% purity works, including bioethanol sold for heating fires and isopropyl alcohol. What you are avoiding is water content, which causes blushing — the shellac dries cloudy. Harmless for most purposes, poor on a sealing coat. The faint residue methylated spirits can leave from its denaturing agents doesn’t matter for boat construction.

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

The pound cut system and what each concentration is for

“Pound cut” is the traditional measure of concentration: pounds of dry flakes dissolved in one US gallon of solvent. In metric, one pound cut is about 119 grams per litre. Multiply the cut number by 119 for grams per litre. The full range and its uses:

For workshop use, a two-pound cut made as 120g of flakes in 500ml of spirits is a useful working quantity for sealing and priming. Mix it in a small glass jar and label it with the date. I keep this made up at all times during a build, for end grain sealing, priming before Le Tonkinois, the odd repair. Having it to hand saves time. Weigh the flakes rather than measuring by volume. Flake density varies enough by grade and moisture that volume measurements are unreliable.

Sealing and priming under natural varnish

Bare timber before its first coat of Le Tonkinois absorbs the varnish unevenly. End grain drinks it, flat grain takes it patchily, the final surface comes out irregular. A 1.5 to 2-pound cut applied to the bare wood, dried fully, then sanded lightly before the next coat, seals the grain and gives a more uniform foundation. The sanding 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. Leave it unsanded and adhesion can fail. Sand it and the problem doesn’t arise.

Worth knowing that Kelly himself is equivocal about shellac as a sealer. He says it costs more than standard varnish and in most cases has no advantage over an ordinary varnish surfacer, that some finishers prefer the latter. The alternative is to skip the shellac and use thinned Le Tonkinois as the first sealing coat, building up from there. For most timber that works well. Where I find shellac earns its place is on end grain specifically, which drinks varnish in a way 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.

Locking fastenings without going permanent

This use matters in natural construction, and the why comes 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 wick moisture into the wood if allowed to. In a wooden hull, water that reaches the frame through a fastening hole promotes decay at exactly the point where the structural loads concentrate. The conventional fixes, synthetic thread-locking compounds and polyurethane sealants, work but are permanent, incompatible with natural varnish systems, and against the principle that every fastening in this hull should come out for maintenance without special equipment.

Shellac solved this for me. Apply a 2.5-pound cut to the screw thread, 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. Thirty seconds per fastening.

Removal is the part that sells it. Work a few drops of methylated spirits into the thread, wait a minute, and the shellac dissolves. The screw extracts without damage to timber or fastening. I’ve done this on cleats and deck fittings that sat through several sailing seasons and they came out cleanly. Nothing toxic, nothing permanent.

One restriction is absolute. Alcohol shellac only for fastenings. Ammonia reacts with copper, bronze, and brass, causing stress corrosion cracking in those metals. Almost all wooden boat fastenings are silicon bronze, brass, or copper, so ammonia-dissolved shellac belongs nowhere near them. This is not a marginal risk. It is a real failure mode that can destroy a fastening invisibly over time. If you want to lubricate the screw during driving as well as seal it, useful in dense hardwoods, rub tallow or beeswax on the thread first, then the shellac. They are compatible.

There is good precedent for shellac as a structural sealing layer in timber. Nathanael Herreshoff, who designed six successive America’s Cup defenders, used shellac-saturated silk as the interlayer between the two skins of his double-planked hulls. The silk acted as sealant and bonding layer at once, keeping water out of the seam and adding coherence to the assembly. The exact formulation isn’t documented, and whether it was alcohol or ammonia shellac is unclear. Given the working time needed during planking, plain alcohol shellac as a sealant-adhesive is the more likely candidate.

Waterproofing paper charts

A satisfying application that doesn’t fit the other categories. Charts, tide tables, and log sheets can be waterproofed with a one-pound cut applied in two thin coats, drying fully between. 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 gives a stiffer result that holds for charts living permanently in a holder but cracks when folded. For notes and log pages that need to survive a wet cockpit, a single thin coat on both sides does it.

Garnet gives the paper a warm amber tint that reads perfectly well and is rather pleasing. Super-blonde if you want the original colour. Ten minutes, start to finish. More reversible than lamination, and more useful than a waterproof case that gets left below.

Ammonia shellac as a light adhesive

When shellac is dissolved in ammonia rather than alcohol, the chemistry changes in ways that matter. Ammonia is itself a solvent for the resin, so no alcohol is needed or wanted, and it reacts with the resin to form ammonium shellacate, a mobile, tacky solution with real adhesive properties beyond those of alcohol shellac. The other 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.

To mix it, place dry flakes in a sealed glass jar and cover with household ammonia, the 10% solution sold as a cleaning product, at roughly three parts ammonia to one part flakes by weight. Seal and leave at room temperature. Dissolution takes several hours. Speed it up by standing the sealed jar in a warm water bath at around 50°C for half an hour to an hour, shaking occasionally. The result is a dark amber, viscous, pungent solution. Work in a ventilated space and do not add alcohol.

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 work is lightweight internal fittings, small hardware and trim, lining fabrics, and minor repairs where the joint carries no structural load. Its advantages there 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 with most natural adhesives. Surface-to-surface and lap bonds in shear are where it performs. And, again, not near fastenings. Ammonia and copper alloys do not mix.

Ammonia shellac and wood flour as a grain filler

One thing I’ve been experimenting with is ammonia shellac as the liquid in a wood flour filler paste. Mix fine wood flour, from the same species if possible or a close match, into a three-pound cut ammonia shellac until you get a thick workable paste. Press it into damaged grain, open knots, small voids, or surface checks, and let the ammonia evaporate fully before sanding back. The result is a filler chemically related to the wood surface, compatible with the shellac primer coats that follow, and biodegradable.

The practical advantage over proprietary fillers is compatibility. Most are acrylic or polyester-based and sit awkwardly in a natural material system. They might 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 of a piece with everything else in the finishing schedule. Ammonia shellac is the right base rather than alcohol shellac because the longer open time lets you work the paste into voids and tool it before it sets. Alcohol shellac skins too quickly for anything beyond the smallest repair. The paste stiffens as the ammonia evaporates, shrinks slightly as it cures, and sands cleanly when dry. Overfill and sand back rather than trying to fill flush. Two applications may be needed on deeper voids.

I’m still gathering evidence on long-term durability under varnish. How the filled areas behave through wetting and drying, whether the filler moves differently from the timber around it. Early results look good. This is work in progress.

Where it sits in the rest of the system

Shellac, casein, and the rubberised bedding compound make a complete natural adhesives and sealants system for skin on frame construction, each with a defined role. Casein takes the structural loads. Shellac seals, primes, locks fastenings, handles light bonding. The rubberised compound beds hardware and pays seams. None requires specialist disposal. All are reversible. All biodegrade. The adhesives hub covers how they fit together and where each is specified in a build, and the boat plans note the application at each joint and fitting.

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: