Collection: Field Notes — Preserving Natural Materials at Sea 

Series Hub: Preserving Wood 

Subject: What spar varnish and polyurethane actually are, how Le Tonkinois sits differently from both, and which failure modes you are choosing between

Le Tonkinois and Spar Varnish vs Polyurethane

There is a spar in my workshop that I have been watching for four years. Half of it was finished with polyurethane — a good quality marine product, applied correctly, three coats. The other half was finished with Le Tonkinois, two coats, no particular care taken in application. The spar has been outside, on racks, exposed to weather, not maintained. Neither half has been touched since the day I finished it.

The polyurethane half failed in the second winter. The film cracked at a knot, lifted at the end grain, and water got under it. By the end of the third year the exposed end had soft fibres where the film had bridged over rather than sealed the end grain, and moisture had been sitting against the wood surface in a sealed channel for two seasons. The Le Tonkinois half is dry. The surface has greyed and the sheen is gone. The wood underneath is intact.

I am not drawing sweeping conclusions from one test spar that was not set up as a rigorous trial. But the failure pattern on the polyurethane side was exactly what the chemistry predicts, and the Le Tonkinois side has so far done what its penetrating-oil character suggests it should do. It is the kind of result that makes you want to keep watching rather than calling it settled.

The Preserving Wood series covers the related treatments. The linseed vs polyurethane note covers the penetrating oil versus film finish question in more detail than this note has space for. The VAKA field notes hub has the broader context.

What Spar Varnish Is

The name comes from spars — masts, booms, gaffs, bowsprits — the round timber poles that carry sail and take the full punishment of UV, salt spray, and constant mechanical flexing. Traditional spar varnish was developed for that specific application, and the formulation reflects it.

At its core, oil-based spar varnish is drying oil cooked with alkyd resin and dissolved in solvent. The oil content is higher than in interior varnishes. More oil means a more flexible cured film — one that can move with seasonal expansion and contraction of the grain without cracking away from the surface. Interior varnishes optimised for hardness crack on exterior surfaces because they cannot accommodate that movement. Spar varnish trades some hardness for the flexibility that exterior wood requires.

Most quality spar varnishes also contain UV absorbers — compounds that intercept ultraviolet radiation before it degrades the surface lignin. The UV degradation sequence covered in the wood failure note — surface oxidation, micro-cracking, moisture entry, rot establishment — is exactly what the UV package in a spar varnish is designed to slow. Whether it slows it indefinitely or merely delays the same sequence is what maintenance schedules are for.

Water-based spar varnish is a different material wearing the same name. Acrylic or acrylic-alkyd resin dispersed in water, drying fast, lower VOC, easier cleanup. The track record in demanding marine applications is shorter than oil-based spar and the flexibility data less impressive. For interior work and light exterior use the practical advantages of water-based are real. For a working spar on a boat in regular salt water use, the oil-based product has the better evidence base. I have not used water-based spar on anything structural and I am not in a hurry to start.

What Polyurethane Is and Why It Fails Outdoors

Polyurethane is a synthetic polymer cured to a hard, abrasion-resistant film. Harder than alkyd spar varnish. More scratch-resistant. Chemically more resistant when intact. On a hardwood floor that takes daily chair-scraping and spilled coffee, these properties are genuinely useful and nothing in the natural materials toolkit competes with them directly.

The problem on exterior wood is the hardness itself. Polyurethane is harder than the substrate it is finishing. Wood moves across the grain as its moisture content changes. A rigid film on a moving substrate responds by cracking — typically at joints, end grain, and any concentration of movement or stress. The crack admits water. The intact surrounding film prevents the water from leaving. What you have created, in a location you cannot see, is a persistently moist environment against the wood surface. The wood failure note covers why that is the condition rot needs.

This is what happened on my test spar. The polyurethane film was intact over most of its surface for two full seasons. At the knot — a point of higher density and different moisture movement from the surrounding wood — it cracked. At the end grain — where I had applied it over bare wood without an oil or shellac primer — it lifted. Those two failure points were enough. By the time the visible damage was obvious, the rot had established behind the film in channels that were structurally inaccessible.

The "spar urethane" or "polyurethane spar" products attempt to address this by adding flexibility to the urethane formulation. They are better than standard polyurethane for exterior use. They are not as good as high-oil spar varnish, because the chemistry of urethane resin does not accommodate wood movement as well as alkyd-oil formulations regardless of additive packages. I would not use any polyurethane product on exterior marine timber. This is not a natural-materials preference. It is a failure-mode preference.

Le Tonkinois

Le Tonkinois is made by a French company of the same name, and has been in production since the late nineteenth century. The name refers to Tonkin — northern Vietnam — from which the tung oil in the original formulation was sourced. The core chemistry is tung oil processed with natural resins, producing a product that sits between a penetrating oil and a film-forming varnish in its behaviour — not fully one or the other, and more useful for that position than it might initially appear.

What the tung oil component does is penetrate into the wood surface before the resin component builds a film above it. The first coat goes in more than it sits on top. Subsequent coats build more at the surface. By the fourth or fifth coat the product is behaving more like a varnish than an oil, but the initial penetration means there is no sharp boundary between treated wood and finish surface of the kind that creates lifting and delamination in pure film finishes.

The cured film is harder than pure tung oil but significantly more flexible than polyurethane. It does not crack at joints in the same way because the oil component in the resin system allows some accommodation of wood movement. When it eventually weathers or wears, it does so gradually and visibly — the surface loses sheen, begins to look dry and slightly grey, before there is any structural indication. This is the maintenance signal you want. Not a crack. Not a lift. Just a surface telling you it is time for another coat.

Recoating is the other practical advantage. Scuff lightly if the surface is glossy, clean, apply a fresh coat. The new material integrates with what remains of the existing finish rather than sitting as a discrete new layer over a degraded old one. This is the fundamental maintenance advantage of oil-resin systems over pure film systems, and it compounds over the life of the boat. A Le Tonkinois-finished surface maintained every two or three seasons with a recoat never accumulates the degraded film build-up that eventually requires stripping on a polyurethane-finished surface.

Le Tonkinois vs Spar Varnish

A quality oil-based spar varnish is a good product and the traditional choice for brightwork has a long track record that is hard to argue with purely on the basis of chemistry. The practical comparison with Le Tonkinois involves a few specific differences.

Traditional spar varnish builds a thicker, glossier film faster. For deep, glass-like brightwork on a boat that is maintained carefully each season and presented well, it achieves an aesthetic that Le Tonkinois in its standard form does not match. If that visual quality is the goal and the maintenance commitment is genuine, spar varnish delivers it.

Le Tonkinois builds more slowly, is less glossy in its natural state, and is more forgiving of imperfect maintenance intervals. A spar varnish surface that has been allowed to weather for two seasons without attention needs cutting back and potentially stripping before a sound maintenance coat can be applied. A Le Tonkinois surface in the same condition can usually receive a maintenance coat applied directly over the weathered surface after cleaning. This difference matters on working boats that get used hard and do not always receive the attention they deserve at the scheduled interval.

Availability is the practical limitation of Le Tonkinois. It is a specialist product from a single manufacturer, available in the UK through a small number of suppliers. Spar varnish is on the shelf of every chandlery. On a passage to somewhere without specialist marine suppliers, the ability to replace or maintain a finish matters. I carry spar varnish as a maintenance material for exactly this reason, even on boats primarily finished with Le Tonkinois.

The Environmental Dimension

When a coating weathers off a hull or spar, what it releases depends on what it is. Polyurethane is a synthetic polymer. As it degrades under UV and mechanical wear, it fragments. Fine enough fragments are microplastic particles. Sanded polyurethane produces them directly. The microplastics note covers what that means in a marine context.

Alkyd spar varnish is a synthetic resin in an oil base. The oil component degrades into organic compounds. The alkyd resin component fragments in ways broadly similar to polyurethane, though the polymer chemistry differs. Neither is clean. Both contribute to the coating load in the water. The true environmental cost of boating note covers this in the broader picture.

Le Tonkinois, based on tung oil and natural resins, comes closer to a genuinely natural coating system than anything else in the film-building varnish category. Tung oil degrades into organic fatty acid compounds. Natural resin components degrade similarly. The environmental profile of the degradation products differs meaningfully from synthetic polymer systems. This is not a zero-impact material — processing and transport have their own costs — but the end-of-life chemistry is different in kind from polyurethane.

Application

Spar varnish application requires a prepared surface, a dust-free environment, and more patience between coats than most boat maintenance schedules reward. Sand to 180 to 220 grit, remove dust thoroughly, apply thin coats with a good quality natural bristle brush. First coat thinned 10 to 15 percent on bare wood. Allow each coat to dry fully — 24 hours minimum, longer in cold or damp conditions — before scuff sanding with 220 to 320 grit and applying the next. Three to four coats for a working build, six to eight for deep brightwork. End grain wants a penetrating sealer before any varnish coat — shellac on bare end grain, or oil — because varnish alone bridges the open vessels rather than sealing them, and will lift there first.

Polyurethane follows similar application logic with one critical addition: intercoat adhesion timing. Oil-based polyurethane has a window within which recoating is reliable, and outside that window — when the previous coat has fully hardened — mechanical scuffing is essential rather than optional to achieve adequate adhesion. Exceeding the window and recoating without adequate preparation is a common source of later delamination. Follow the manufacturer's instructions on recoat timing more carefully than with spar varnish.

Le Tonkinois is the most forgiving of the three to apply. The oil component self-levels well. Brush marks flow out. The extended open time gives more working latitude in warm weather than fast-drying synthetic products. Apply thinly — the temptation to build quickly with heavy coats produces a gummy result that takes much longer to cure than thin coats properly applied. Clean brushes with white spirit or citrus solvent. Below 10°C the product thickens and should not be applied — the cure slows and the penetration reduces. In cool weather warm the tin in water for ten minutes before use.

The Test Spar, Continued

I will keep watching the workshop spar. The Le Tonkinois side needs a maintenance coat — it is grey and dry-looking and I should have done it last autumn. The polyurethane side is a different problem, requiring assessment of how far the softening at the end grain has progressed before deciding whether the timber beneath is still sound enough to work with. That assessment will happen this spring.

The result will not be a controlled trial. Too many variables are uncontrolled. But it is the kind of observation that either confirms or complicates the theoretical picture, and both outcomes are useful. If the Le Tonkinois side deteriorates after maintenance in a way that the theory does not predict, that is worth knowing. If the polyurethane failure pattern continues to match what the chemistry suggests, that is worth documenting too.

This is how most of the knowledge in these notes has developed. Not from formal trials but from watching specific things in specific conditions and asking why they behaved the way they did.

Sources: 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). Le Tonkinois product documentation at letonkinois.fr.

At VAKA I design regenerative boats for myself and then for people who want boats that can be maintained rather than replaced. Plans at VAKA Boatplans; the full knowledge base at Field Notes. Notes.