Treating, Coating and Preserving Wood —The Overview

Collection: Field Notes — Preserving Natural Materials at Sea 

Series Hub: Preserving Natural Materials at Sea 

Subject: A guide to the notes in this series — how the investigation started, where it has led, and what remains unresolved

This series started with a rotting boat and a question I could not satisfactorily answer.

The boat was not mine. I was helping with a friend look at a boat he wanted to buy, and what I found under the trim was timber that had been failing for two seasons behind paint that looked intact. The paint had bridged the end grain rather than sealed it, moisture had found the gap at the paint edge and stayed there, and the rot that followed had worked in conditions that the surface appearance had completely concealed. The boat had been maintained. It had been maintained in a way that created the failure rather than preventing it, and no one involved had understood what was happening until the spike went in.

That discovery sent me into the wood preservation literature with more urgency than I had previously brought to it. What I found there was useful but not always coherent — reference books that contradicted each other on specific points, traditional practices with no clear mechanistic account of why they worked, and a market of products whose labels bore little reliable relationship to their contents. The notes in this series are my attempt to work through that material honestly, test what can be tested at small scale, document what I find, and be clear about the line between what I know and what I am still uncertain about.

The series is not finished. Neither is the investigation.

The Preserving Natural Materials at Sea hub covers how this series sits alongside the canvas, rope, hull coatings, and oilskins work. The VAKA field notes hub has the context for why building in natural materials rather than synthetic ones is the premise all of this starts from.

Starting with Failure

The first note in the series, Why Wood Fails at Sea, is where the investigation actually began. Not with treatments, but with mechanisms. Wood rot is biological, not chemical or mechanical — fungi consuming structural components as food, requiring moisture above 20%, oxygen, and temperature above roughly 5°C. Remove any one and the rot stops. UV damage matters primarily as a rot enabler rather than a standalone problem: it opens micro-cracks in the surface that allow moisture entry, which is where the damage actually starts. End grain fails by a different mechanism from face grain and needs to be addressed differently.

The practical implication I was slowest to absorb: drainage, ventilation, and correct drying prevent more damage than any preservative system can reverse. Treatment is the last line of defence. The lines before it matter more. I had been thinking about wood preservation primarily as a surface treatment problem when it is primarily a moisture management problem that surface treatment supports.

Species — What You Are Starting With

The naturally waterproof wood note covers what natural durability actually rests on — extractive chemistry in the heartwood, cell structure, the tyloses in white and European oak that make it effectively impermeable to liquid regardless of extractive content, the thujaplicins in western red cedar that provide biological resistance beyond what density alone would predict. It also covers the species used at VAKA specifically: oak for primary structure, western red cedar for planking, larch for exposed and semi-exposed structural elements, spruce for spars, ash for oars and impact-load members, red grandis as an increasingly useful plantation-grown alternative.

The conclusion I keep returning to: species choice changes the maintenance commitment more than any other single decision. A naturally durable species in the right location needs maintenance; the wrong species in the wrong location needs rescuing. Getting this right at the design stage is more effective than any treatment system applied to timber that was wrong for the application.

The durability classifications in the reference literature are averages, and they apply to heartwood. Sapwood in most species — including oak, cedar, and larch — has essentially no biological resistance regardless of what the heartwood does. This is where I have seen the most significant practical failures in timber selection, including my own: pale outer bands on planks that were specified as durable species but contained enough sapwood to fail at the pace of untreated softwood.

The Oil Family

Three notes cover the linseed oil family in enough depth to be useful rather than just introductory. Raw vs Boiled vs Stand Oil covers what those three products actually are — raw oil being pressed flax seed with nothing added, commercial BLO being raw oil with metallic drier salts rather than anything boiled, stand oil being heavily thermally polymerised oil that builds a harder surface film. The gap between "boiled linseed" on the label and what is actually in the tin was the kind of discovery that made me want to look more carefully at other products I had been accepting at face value.

The homemade heat-bodied linseed note covers the process of making a genuinely heat-processed oil without metallic driers — the temperatures, the equipment, the fire safety considerations that are not theoretical, and an honest account of what the result actually achieves compared to commercial BLO. I have been using it for four seasons. It works. Whether it is definitively better than commercial BLO or just different without synthetic additions is a question I have not resolved to my satisfaction.

Tung oil has its own note because the adulteration problem in the market is significant enough to deserve specific attention. Most products sold as tung oil are not tung oil. The glass test — leaving a small amount on glass to cure and observing the result — is the practical verification I now do before using any new tung oil product on actual work. Verified pure tung oil performs as the chemistry predicts: faster cure than linseed without metallic driers, better initial water resistance, non-yellowing. Adulterated products do not.

Danish Oil and the Blend Problem

The Danish oil note covers a product category that I had misunderstood for longer than I want to admit. Danish oil is an oil/varnish blend — approximately one part drying oil, one part alkyd resin, one part petroleum solvent — not a premium penetrating oil. The varnish content that makes it useful for interior woodworking is exactly what makes it the wrong product for exterior and marine surfaces: alkyd resin forms a surface film that cracks and lifts under seasonal wood movement, trapping moisture behind it in the same failure mode as polyurethane.

Applied to gunwales on a boat I was helping maintain — because it was in the locker — I watched it produce exactly that failure pattern over two seasons. I cannot prove causation from one observation, but the pattern was consistent with what the chemistry predicted, and I no longer use Danish oil on anything that goes outside.

Wax

The beeswax on wood note covers where beeswax earns its place and where it does not, with specific recipes for interior paste, exterior carnauba blend, and the combined oil-wax treatment that consolidates and finishes in a single warm application. The key finding from testing was the application method: heated wax on warm wood produces genuinely different penetration from cold wax on cold wood, and the difference in durability of the two results is significant enough that application method is most of the result.

The question I have not resolved is the long-term comparative durability of a good beeswax-carnauba treatment versus a well-maintained linseed oil treatment on the same exterior timber surface. My observation suggests they address different layers of the same problem — oil for structural moisture management within the fibre, wax for surface water shedding — and are more complementary than competitive. But I have not run a rigorous comparison.

Boat Soup

The boat soup note covers the blended treatment that has been keeping wooden working boats alive for centuries. Linseed oil, Stockholm tar, turpentine — three parts oil to two parts tar to one part turpentine as a working base. The combination does things no single ingredient manages: deep penetration from the thinned blend, moisture exclusion from the polymerised oil, biological resistance from the tar's phenolics.

I started using boat soup before I understood why it worked. The working backwards — understanding the chemistry clearly enough to adjust the blend for specific applications and conditions — changed how I use it. The tung oil variant, made with verified pure tung oil, produces noticeably better water resistance and surface character for above-waterline visible work. The pine resin variant cures faster and harder but is less flexible at joints — useful in some applications and not others. For saltwater skin-on-frame hulls, boat soup on the frame before skinning remains the primary treatment. For freshwater canoes in sheltered conditions, I have used plain tung oil and the results after three seasons have been adequate, though I hold that conclusion as provisional rather than established.

Stockholm Tar

The Stockholm tar note covers the material in detail: the production by slow pyrolysis of pine heartwood, the phenolic compounds that provide biocidal action, the distinction from coal tar that matters and is frequently confused in the market. The application failure I had on a first use — too thick, too cold, persistently tacky — taught me more about the material than the correct application would have, because it forced me to understand what the tar needed in order to work rather than just following a procedure.

Larch under regular annual pine tar treatment behaves like timber two durability classes above its classification. Whether this is the tar's contribution or a favourable interaction between larch's own extractive chemistry and the tar's phenolics, I have not found a clear mechanistic account. The observation is consistent. The explanation is not settled.

The Film Finish Comparison

Le Tonkinois and spar varnish versus polyurethane is documented through a specific observation: a test spar with one half finished in polyurethane, one half in Le Tonkinois, left outside without maintenance for four years. The polyurethane failed in the second winter — cracked at a knot, lifted at end grain, rot established behind the film. The Le Tonkinois half is dry, grey, and needs a maintenance coat. The failure pattern on the polyurethane side matched exactly what the chemistry predicts. The observation is not a controlled trial, but it is the kind of specific result that either confirms or complicates theory, and in this case it confirmed it.

The polyurethane versus shellac note covers a comparison that is usually framed as alternatives when the two materials mostly suit different applications that do not overlap. Polyurethane for high-wear interior surfaces. Shellac for dry interior joinery that benefits from repairability over time. The cabinet in my workshop, repaired a dozen times in eight years without stripping, is the practical evidence for shellac's long-term maintenance advantage on interior work.

Shellac, End Grain, and the Sequence That Matters

The shellac note covers grades, solvents, and the science behind waxed versus dewaxed water resistance — the counterintuitive conclusion that dewaxed outperforms waxed for sustained moisture resistance despite feeling less hydrophobic on the surface. It also covers the applications that surprised me when I started using shellac seriously: thread-locking, woven seat treatment, chart waterproofing, and the Henley's pitch-shellac-rubber compound that I have not yet made but intend to.

The end grain treatment note starts with the beam end I probed three years ago — paint intact, timber soft through the full depth — and works through what actually prevents that failure. The sequence that emerged from the investigation: shellac as a standalone capillary shut-off on enclosed end grain, hot linseed oil as the penetrating treatment on accessible end grain, shellac as a tannin-sealer and surface consolidant on face grain where relevant. Not oil over shellac, which was a mistake in an earlier version of this note that the subsequent investigation corrected.

The linseed versus shellac note pulls the comparison together, including the Wooden Boat Magazine dinghy account that illustrates something important about failure modes: shellac fails by disappearing, which leaves dry wood; varnish fails by lifting and trapping, which leaves concealed rot. The dinghy survived not because shellac provided better protection but because it degraded in a mode that left the wood accessible rather than sealed.

The Metal Salt Work

The zinc chloride and Burnettizing note covers Sir William Burnett — naval surgeon, veteran of Trafalgar, and sufficiently entrepreneurially minded to nearly wreck his professional reputation promoting a zinc chloride patent in the 1830s — and the preservation chemistry he was working with, which remains sound. The mordanting framework that underlies all metal salt canvas and timber treatment, the spectrum from zinc through iron and copper to alum, and the practical sequences for small-scale use without industrial pressure vessels. Several of those sequences are documented as logical extrapolations rather than tested protocols. Testing is ongoing.

What Remains Open

There is more here that is uncertain than the series might appear to suggest, and I want to be clear about that rather than letting the accumulated detail imply more settled knowledge than I have.

The long-term comparative durability of heat-bodied linseed against commercial BLO under sustained marine conditions is not something I have tested rigorously enough to state with confidence. Four seasons of observation is a start, not a conclusion.

The larch-tar interaction — why tarred larch performs so far above its durability classification — is a consistent observation without a clear mechanistic account. I would like to understand it better than I do.

The rice bran wax experiments in the canvas series are too recent to have produced results worth drawing conclusions from.

The Henley's compound is unmade. The metal salt canvas treatment sequences documented in the Burnettizing note are extrapolations from chemistry rather than documented results from extended field trials.

These are not gaps I am embarrassed about. They are the shape of an ongoing investigation rather than a completed reference. The notes that have results document them; the notes that have hypotheses say so. The distinction matters and I try to maintain it throughout the series.

The series starts there. Why Wood Fails at Sea is the foundation note — rot biology, UV degradation, mechanical stress, and the way these failure modes compound each other on a boat in regular use. It is worth reading before anything else, not because the treatments make no sense without it, but because understanding what you are actually trying to prevent changes how you think about maintenance. Most wood failures on wooden boats are not bad luck. They are predictable, and they were preventable.

Two things are worth establishing before getting into specific materials. First: the protection hierarchy. Bob Flexner's Understanding Wood Finishing places finishing materials in a clear sequence from least to most protective — wax at the bottom, then oil, then shellac, lacquer, varnish, and film-forming finishes at the top. The right material is not always the most protective one. It is the one whose failure mode is appropriate for the application. A wax finish that fails visibly and can be renewed in minutes is sometimes more valuable than a varnish that lasts longer but fails catastrophically and invisibly. Second: application conditions are not secondary considerations. Research by E. Brandt and T. Lading surveying real-world linseed oil treatments found outcomes ranging from failure within two years to excellent condition after eight-plus years — the difference almost entirely accounted for by workmanship and application conditions, not product quality. Wood below 12–15% moisture content, temperatures above 5–10°C, thin coats, and proper inter-coat drying are not optional refinements. They are the treatment.

Species First

Naturally Waterproof Wood covers what makes some timbers inherently more resistant to decay and moisture than others — extractive chemistry, cell structure, tyloses, dimensional stability — and works through the species used at VAKA specifically: white and European oak, western red cedar, larch, spruce, ash, and red grandis. Choosing the right species for the right application is the most effective preservation decision you can make, well before any treatment is applied. A naturally durable species in the right location needs maintenance; the wrong species in the wrong location needs rescuing. No treatment compensates fully for a species mismatched to its environment.

Penetrating Oils — the Workhorse Family

Penetrating oils are the foundation of the natural materials preservation system. They work by getting into the wood rather than sitting above it — distributed through the surface fibre structure, curing in place, leaving no discrete film to crack or trap water. The failure mode is gradual depletion and visible surface greying rather than film failure and concealed rot, which is why they suit exterior and marine wood in a way that film finishes do not. Miha Humar and Bostjan Lesar's laboratory and field research confirmed the mechanism: oil-treated timber allows the wood to dry after precipitation, and the reduction in average moisture content is sufficient to slow fungal attack significantly. Their data also confirmed that tung oil outperforms linseed oil on water exclusion and fungal resistance across all test conditions — which the treatment hierarchy below reflects.

Raw Linseed, Boiled Linseed and Stand Oil unpacks what those three products actually are and how they differ — because the distinctions matter practically and the labelling in the market is unreliable. Raw linseed penetrates most deeply but dries soft and tacky and should not be used as a standalone finish — Ashmun Kelly's The Expert Wood Finisher is emphatic on this, and the Brandt and Lading field survey confirms that many treatment failures originate in exactly this mistake. Commercial boiled linseed oil adds metallic driers to accelerate cure; they work, but cobalt, manganese, and lead compounds are not without environmental concern, and anyone building with ecology as a genuine priority should be aware of what is in the tin.

DIY Heat-Bodied Linseed Oil covers the process of genuinely heat-polymerising raw oil at home without metallic driers — the cleaner product for anyone who wants complete transparency about what is going on the wood and eventually into the water around it.

Tung Oil earns its own note partly because it performs differently from linseed — faster-curing without metallic driers, more water-resistant from the first coat, non-yellowing — and partly because the adulteration problem in the market is significant enough that buying the wrong product and expecting tung oil's properties is a reliable route to disappointment. Humar and Lesar's research specifically found tung-oil-treated spruce retaining significantly lower long-term water uptake than linseed-treated spruce, where linseed-treated specimens eventually converged with untreated controls. For exposed structural members and for oily tropical hardwoods where linseed penetrates poorly, pure tung oil from a verified supplier is the better-performing oil. Most products sold as tung oil are not pure tung oil. The note covers what pure tung oil actually is, where to source it, and how to tell the difference.

Danish Oil rounds out the oil section by examining what is probably the most widely sold wood treatment in UK hardware shops — and why it behaves differently from pure oil finishes in ways that matter on boats. Flexner classifies Danish oil accurately as a wiping varnish: an oil/varnish blend with petroleum solvent, not a penetrating oil. Its varnish content builds a partial surface film that is useful on dry interior furniture and a liability on any exterior or marine surface subject to seasonal wood movement. The mechanism — varnish film cracking at joints, water trapped behind intact-looking surface, concealed rot — is the same failure mode as polyurethane on exterior wood, at smaller scale and slightly later. Do not use Danish oil on exterior or marine surfaces.

The three-way comparison of tung oil, linseed oil, and Danish oil puts all of this in a single direct note for anyone who wants the decision laid out simply.

Wax and Blended Treatments

Beeswax on Wood covers where beeswax earns its place — and where it does not. Both Flexner and Kelly are explicit that beeswax is not a moisture barrier: Flexner states it is the least protective of all finishes, the closest thing to no finish at all on the wood; Kelly notes that wax finish is easily affected by water and dampness and should not be allowed to sit on a waxed surface. The water beading visible on a freshly waxed surface is a surface phenomenon only, not an indication of film integrity below it. Beeswax belongs on interior furniture and handled surfaces where its tactile character, surface lubrication, and renewability are genuine advantages — not on any surface where moisture management is the priority.

Where beeswax is most useful is in combination: cured linseed oil providing the penetrating structural treatment, beeswax over the top providing surface character and ease of renewal. Ashmun Kelly's own formula for a combined oil-wax treatment — beeswax and raw linseed oil melted together with turpentine — is the historical precedent for the same combined approach used in the oilskins and waxed cloth system, where a linseed and wax blend has treated fabric for protective outerwear for generations. The principle translates directly from cloth to timber. For interior furniture specifically, Kelly also notes that a thin shellac sealer under the beeswax gives a more maintainable long-term surface than wax on bare wood — easier to clean down and renew without disturbing the wood beneath.

Boat Soup is where the blending logic reaches its natural conclusion for marine service. Plain boiled linseed oil on a saltwater boat is the starting point, not the destination — it provides moisture management and some fungal resistance proportional to penetration depth, but it is not sufficient biological protection for timber in continuously wetted marine service. The combination of linseed oil, Stockholm tar, and turpentine does things no single ingredient manages alone: deep penetration from the oil, biological resistance from the tar's phenolic compounds, and workability and penetration-driving from the turpentine. For freshwater canoes in sheltered use, pure tung oil is adequate. For regular saltwater service — and particularly for lashed natural cordage construction where the cordage itself is vulnerable — the tar component of boat soup earns its place unambiguously. This is the default exterior wood treatment for all VAKA saltwater builds.

Stockholm Tar gets its own dedicated note because it deserves one. The biology of its preservative action, the crucial distinction between genuine pine tar and coal tar derivatives (which matters and is frequently confused in the market), application method, and the maintenance programme that keeps tarred timber performing well over the long term are all covered there. Stockholm tar is a genuine biocide with centuries of documented use on wooden boats — not a romantic material choice, but a chemically active one.

Surface-Building Finishes — Where Film Finishes Belong and Where They Do Not

The surface-building finishes occupy the last section of the series, and the notes in this group are as much about what not to use as what to use. Flexner's protection hierarchy places these materials above oils in terms of surface hardness and abrasion resistance — which is precisely why they suit interior high-wear surfaces and precisely why they fail on exterior wood that moves.

Le Tonkinois and Spar Varnish vs Polyurethane makes the case that polyurethane's hardness — the property most often cited in its favour — is exactly what makes it a poor choice for exterior marine wood. Flexner is specific: polyurethane is too rigid for exterior wood. Hard and rigid means brittle failure at joints and end grain, water trapped behind intact-looking film, rot developing invisibly before the surface shows any sign of trouble. Spar varnish handles this better because its higher oil content allows the film to flex with seasonal wood movement. Le Tonkinois handles it better still, sitting between a penetrating oil and a surface varnish in a way that fails gradually and visibly rather than catastrophically and late.

Linseed Oil vs Polyurethane makes the broader case about why film finishes and penetrating oil finishes suit almost entirely different applications — and why the conventional wisdom that polyurethane is "better" than oil reflects laboratory surface-hardness testing rather than real-world performance on wood that moves, weathers, and needs maintenance over decades. Brandt and Lading's field survey of real buildings treated with linseed oil paints found no examples of wood in poor condition beneath essentially intact linseed treatment — in direct contrast to latex paint, where concealed rot behind intact-looking film is a documented and common failure mode.

Shellac as Waterproofer, Sealer and Finish goes deepest into a material that tends to be underestimated. Grades, solvents, the counterintuitive science of why dewaxed shellac outperforms waxed for sustained water resistance despite feeling less hydrophobic, and the range of applications beyond conventional wood finishing — thread-locking, woven seat treatment, chart waterproofing, ammonia shellac as a grain filler, and the pitch-shellac-rubber compound from Henley's 1907 formulas that predates synthetic rubberised coatings by half a century. Kelly's additives section — Venice turpentine for improved flow and toughness, camphor for flexibility, lavender oil for frilling, oxalic acid for brightening — represents accumulated workshop knowledge from the period when shellac was the dominant interior finish and has essentially vanished from modern finishing literature. The Shellac Glue, Filler, Sealer and Threadlock note covers the adhesive and gap-filling applications specifically.

Linseed Oil and Shellac — Sequences and Failure Modes is the practical note for anyone using both materials on the same build, which at VAKA is nearly always the case. The sequencing logic is not arbitrary — oil on face grain first because shellac seals the surface and oil cannot penetrate a sealed surface; shellac on end grain as the complete treatment because it closes capillaries that no oil coat will reopen; one medium coat of shellac under oil varnish maximum, because two coats produces too brittle a body and the chipping failure that follows is a coat-count problem, not a materials problem. Kelly is emphatic on each of these points.

Specialist Materials

Zinc Chloride, Burnettizing and Other Metal Salt Preservatives covers the chemical consolidant approach to early-stage rot arrest — where organic oil and wax treatments are the maintenance system for sound timber, and metal salt treatments are the intervention for timber that has already begun to fail.

End Grain Treatment is probably the single most neglected maintenance task on wooden boats, and the note is one of the most important in the series. Cut ends fail first, fail fastest, and fail in locations that are often inaccessible once the boat is built. Getting end grain treatment right at the build stage — hot linseed oil for penetration where the end grain is open and exposed, dewaxed shellac as the capillary sealer for enclosed structural joints, and where relevant a tar-thinned treatment before joints are closed — is a maintenance programme for the life of the hull rather than a job to revisit annually. On a skin-on-frame build, what is done to rib ends, lashing points, and scarf joints before skinning is all that will ever be done to them.

The Environmental Dimension

The Preserving Natural Materials at Sea collection that this series sits within exists because the choice of finishing material is not neutral, and the difference ends up in the water. Synthetic polymer coatings — polyurethane, Danish oil's alkyd resin content, petroleum-solvent products — fragment as they weather under UV and mechanical wear. Some of those fragments qualify as microplastic particles; sanded synthetic coatings produce them directly. The fibreglass disposal problem is the extreme version of this argument, but synthetic coating systems on wooden boats are a quieter version of the same story.

The natural system — heat-bodied linseed oil, pure tung oil, Stockholm tar, beeswax, shellac, cutch — consists of materials that degrade into organic compounds that re-enter the biological cycle rather than accumulating in it. This is not a marginal environmental distinction. Maintaining a wooden boat with natural materials over its working life is genuinely different from the alternative, in ways that persist in the marine environment long after the boat itself is gone.

Reading Order

For anyone coming to the series fresh, the most useful reading order is:

Why Wood Fails at Sea → Naturally Waterproof Wood → End Grain Treatment → Linseed Oil → Linseed Oil and Shellac → Boat Soup → Stockholm Tar → Shellac → Linseed Oil vs Polyurethane → Beeswax and Linseed → Tung Oil vs Linseed vs Danish Oil → Teak Oil vs Linseed.

For anyone maintaining an existing boat rather than building from scratch, Boat Soup and End Grain Treatment arre probably the most essential, then the relevant treatment notes for whatever surfaces are due attention.

Sources underpinning the series: Ashmun Kelly, The Expert Wood Finisher (1921). Bob Flexner, Understanding Wood Finishing (2005). Miha Humar and Bostjan Lesar, Efficacy of linseed- and tung-oil-treated wood against wood-decay fungi and water uptake, International Biodeterioration & Biodegradation (2013). E. Brandt and T. Lading, Linseed Oil Paint As An Alternative To Wood Preservatives, 9th DBMC Conference (2002). Henley's Twentieth Century Book of Formulas, Processes and Trade Secrets (1907), full text at archive.org.

At VAKA I design and test build skin-on-frame boats built in the materials covered in these notes — maintained rather than disposed of. Plans at VAKA Boatplans; the full knowledge base at Field Notes.