Le Tonkinois and Spar Varnish Vs Polyurethane

Collection: Field Notes — Preserving Natural Materials at Sea

Subject: What spar varnish and polyurethane actually are, how they differ, and why Le Tonkinois sits in a category of its own

There is a version of this comparison that gets written a lot on woodworking forums, and it goes roughly like this: polyurethane is harder and more scratch-resistant, spar varnish is more flexible and better for outdoor use, pick whichever suits your project. That is not wrong exactly, but it skips over most of what is actually interesting about the question — and it leaves Le Tonkinois out entirely, which is where things get genuinely useful for anyone building or maintaining wooden boats.

The real comparison is not just about hardness and flexibility. It is about how different coating chemistries fail, what that failure looks like, what it costs to fix, and what ends up in the water when a coating weathers off a hull or a spar. Those are the questions worth answering before picking up a tin.

The Preserving Wood notes cover the full range of surface treatments used at VAKA. The VAKA field notes hub has the broader context for why material choices matter beyond the workshop.

What Spar Varnish Actually Is

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

At its core, oil-based spar varnish is a combination of drying oils — historically linseed, tung, or a blend of both — cooked with alkyd resin and dissolved in a solvent, typically mineral spirits or naphtha. The oil content is higher than in interior varnishes, which is the key to its behaviour. More oil means more flexibility in the cured film. A spar varnish coat can move with the wood as it expands and contracts through seasonal moisture cycles without cracking away from the surface. Interior varnishes, formulated for hardness rather than flexibility, crack on exterior surfaces because they cannot accommodate that movement.

Most quality spar varnishes also contain UV absorbers — compounds that intercept ultraviolet radiation before it reaches the underlying timber. UV breaks down lignin, opens the wood surface, and initiates the sequence that leads to rot taking hold. A spar varnish with a decent UV package is genuinely slowing that process, not just making the wood look good.

Water-Based Spar Varnish

Water-based spar varnish is a different material wearing the same name. The resin system is acrylic or acrylic-alkyd hybrid rather than the traditional oil-alkyd, and the solvent is water with co-solvents rather than mineral spirits. It dries faster, has lower VOC content, and cleans up easily — all genuine practical advantages. The trade-off is that it is generally less flexible than oil-based spar, penetrates less into open grain, and has a shorter track record in demanding marine applications. For interior woodworking and light exterior use it performs well. For a working spar on a boat in regular salt water use, the traditional oil-based product has the better long-term evidence base.

What Traditional Spar Varnish Is Not

Spar varnish is not a penetrating finish. It forms a surface film — it sits on top of the wood rather than going into it. This matters for maintenance, because a film finish that has been damaged, cracked, or significantly weathered needs to be cut back or stripped before recoating. You cannot simply brush a fresh coat over a degraded film and expect it to bond reliably. Spar varnish maintenance done properly is not especially difficult, but it is periodic and it involves preparation. A varnished spar on a boat in regular use wants attention every season — light sand and one or two fresh coats — and a fuller strip and rebuild every several years depending on exposure.

Polyurethane — What It Is and How It Differs

Polyurethane varnish is a synthetic resin system based on urethane polymer chemistry. It has been around since the mid-twentieth century and displaced traditional varnish in a lot of applications because it does some things considerably better: it is harder, more abrasion-resistant, and in ideal conditions more chemically resistant than alkyd spar varnish. Urethanes are harder than oil-alkyd films by a significant margin, which is why polyurethane became the standard finish for hardwood floors and high-wear interior surfaces — applications where scratch resistance matters more than anything else.

Oil-based polyurethane uses a petroleum-derived resin system dissolved in mineral spirits, similar in application to traditional spar varnish but with a different cured chemistry. Water-based polyurethane substitutes an acrylic-urethane dispersion for the oil-based resin, drying faster and with lower odour. Polyurethane spar or urethane spar products attempt to combine the hardness of urethane chemistry with the flexibility additives needed for exterior use — the label "spar urethane" or "polyurethane spar" is the marketing signal that the manufacturer has tried to address the fundamental flexibility problem.

The Flexibility Problem

The fundamental issue with polyurethane on exterior and marine wood is that urethanes are harder than the substrate they are protecting. Poly varnish does not flex with seasonal wood movement the way a high-oil spar varnish does. As the timber expands in wet conditions and contracts as it dries, a rigid polyurethane film responds by cracking — usually at joints, end grain, and any place where two faces meet at an angle. Water enters the crack, gets behind the film, and is then effectively trapped. What follows is the same sequence described in the Danish oil notes and the tung oil notes: rot developing behind an intact-looking surface, invisible until it is already a structural problem.

This is not a theoretical failure mode. It is what actually happens to polyurethane-coated exterior woodwork — decks, spars, brightwork — after two or three seasons of real outdoor exposure. The finish looks fine from a distance and is quietly failing at every joint.

Polyurethane tends to fail in this cracking-and-trapping mode more aggressively than traditional spar varnish, even spar urethane products specifically formulated for flexibility. The underlying chemistry simply does not accommodate wood movement as well as a high-oil formulation, and the harder the urethane system, the more brittle the failure when it comes.

Oil-Based vs Water-Based Polyurethane

Oil-based polyurethane is more flexible than water-based and has better penetration into open grain on the first coat. It takes longer to dry between coats — typically 24 hours minimum — and has higher solvent content. Water-based polyurethane dries fast, is clear and non-yellowing, and is easier to clean up. It is less flexible and builds a thinner film per coat. For interior work where rapid recoating matters and movement is minimal, water-based polyurethane is perfectly adequate. For anything outside, the flexibility argument tilts toward oil-based, though neither performs as well on exterior wood as a properly maintained oil-based spar varnish.

Le Tonkinois — The Case for a Different Category

Le Tonkinois is a French product, made by the company of the same name, based on tung oil polymerised with natural resins. It has been in continuous production since the late nineteenth century and has a following among traditional boatbuilders, furniture restorers, and wooden boat owners that is disproportionate to its profile in mainstream finishing circles. People who have used it seriously tend to keep using it.

The name refers to Tonkin — the northern Vietnamese region from which the tung oil used in the original formulation was sourced. The chemistry has evolved since the nineteenth century but the core remains: high-quality tung oil processed with natural resin to produce a product that sits between a pure penetrating oil and a film-forming varnish in its behaviour.

What It Actually Does

Le Tonkinois penetrates into the wood surface rather than sitting entirely on top of it — this is the tung oil doing its job, as the tung oil notes explain. But it also builds a surface film over multiple coats, giving it more protection than a pure penetrating oil and a finish that responds to light sanding and recoating in the same way a traditional varnish does. The result is a product that combines the deep grain nourishment of an oil with surface protection closer to a varnish — without the brittle failure mode of polyurethane or the film-only approach of a standard spar.

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 does eventually weather or wear, it does so gradually and visibly — the surface loses its sheen and begins to look dry before it fails structurally, which is the maintenance signal you want. It does not delaminate catastrophically. It does not trap water behind an intact-looking film.

Recoating is straightforward. Light cleaning, light scuff if needed, and a fresh coat applied directly to the existing finish. This is the maintenance model that makes sense for a boat or any outdoor woodworking project in regular use — incremental attention rather than periodic strip-and-rebuild.

Le Tonkinois vs Spar Varnish

A quality oil-based spar varnish is a good product and the traditional choice for spars and brightwork has a lot going for it. The comparison with Le Tonkinois comes down to a few practical differences.

Traditional spar varnish builds a thicker, glossier film faster, which appeals if the visual result — deep, glass-like brightwork — is part of what you are going for. It is also more widely available, which matters when you are on passage somewhere and need a tin off a chandlery shelf rather than waiting for a specialist order.

Le Tonkinois builds more slowly and is less glossy in its natural form, though the company produces a gloss variant. What it offers in return is better wood nourishment from the oil component, a more forgiving recoating schedule, and failure characteristics that are gradual rather than sudden. For a working boat that gets used hard and maintained practically rather than cosmetically, those are real advantages. For show brightwork on a boat that lives in a marina and gets detailed attention every season, a good spar varnish and Le Tonkinois are close enough in performance that the choice is largely personal preference.

Both are categorically better choices for exterior marine wood than any polyurethane product, for the reasons already covered.

The Environmental Dimension

This is where the comparison extends beyond performance and into territory that matters for anyone building with natural materials.

When a synthetic coating weathers off a hull or a spar, what it releases depends on what it is made of. Polyurethane — both oil-based and water-based — is a synthetic polymer. As it degrades under UV and mechanical wear, it fragments. Some of those fragments are fine enough to constitute microplastic particles, which enter the water and do not biodegrade. Sanding polyurethane releases particles directly. Antifouling paints based on synthetic binders follow the same logic. The true environmental impact of boating includes this — the coating system on a boat is not neutral background detail.

Oil-based spar varnish occupies an intermediate position. Alkyd resins are synthetic, derived from petroleum, and fragment in similar ways to polyurethane when they degrade. The oil component — if it is genuinely linseed or tung oil based rather than a pure synthetic alkyd — degrades into organic compounds. The environmental picture is mixed rather than clean.

Le Tonkinois, being based on tung oil and natural resins, comes closer to a genuinely natural coating system than anything else in the film-forming varnish category. Tung oil breaks down into organic compounds. Natural resin components do the same. This is not a zero-impact product — processing and transport have their own footprint, and no coating is entirely without consequence — but the degradation profile of a tung-resin system is meaningfully different from a synthetic polymer. For anyone trying to build and maintain a boat in natural materials, that difference is part of the design brief.

The fibreglass disposal problem is the most dramatic version of this argument, but coating systems on wooden boats are a quieter version of the same question: what happens to this material at the end of its service life, and where does it go in the meantime?

Application — What to Expect from Each

Spar varnish application is straightforward on a prepared surface. Sand to 180–220 grit, remove dust thoroughly, apply thin coats in a dust-free environment with a good quality natural bristle brush or foam roller. The first coat on bare wood benefits from thinning 10–15% to improve penetration. Allow each coat to dry fully — typically 24 hours minimum, longer in cold or damp conditions — before scuff sanding with 220–320 grit and applying the next. Three to four coats is a reasonable base build on a new project; up to six or eight for deep brightwork. The end grain wants an oil or penetrating sealer before any varnish coat is applied — varnish alone bridges end grain rather than sealing it and will lift there first.

Polyurethane follows similar application logic but is less forgiving. Intercoat adhesion can be tricky — if the previous coat has fully cured hard before recoating, mechanical scuffing is essential rather than optional, and some oil-based polyurethane systems require a specific intercoat time window within which adhesion is reliable. Exceeding that window and applying over a fully hardened previous coat risks adhesion failure. 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 readily, and the extended open time gives more working latitude than fast-drying synthetic products. Apply thinly — the temptation to build quickly with thick coats is worth resisting — allow each coat to cure fully, and clean brushes with white spirit or the manufacturer's recommended solvent. Maintenance recoating does not require stripping or heavy sanding: cleaning the surface, light abrasion if the finish is glossy, and a fresh coat will bond reliably to a well-maintained existing film.

In cold weather, all three products slow down. Le Tonkinois and spar varnish are both workable down to around 10°C with some reduction in flow. Polyurethane in cold conditions thickens noticeably and the recoat window tightens further. None of them should be applied to a surface below 10°C if a reliable result matters.

The Practical Summary

For interior woodworking and surfaces that stay dry — cabin joinery, tool handles, interior furniture — polyurethane is a legitimate choice where hardness and abrasion resistance are the priority. It is widely available, applies easily, and performs well in conditions it was actually designed for.

For exterior wood on a boat, or any wood subject to repeated wetting and UV exposure, traditional spar varnish outperforms polyurethane on the grounds that matter: it is more flexible, it fails more gracefully, and it is easier to maintain without periodic complete stripping. The coating design has been field-tested for a very long time in exactly this application.

Le Tonkinois goes a step further: better wood nourishment from the oil base, more forgiving maintenance, a more natural material profile, and failure behaviour that gives you time to respond rather than presenting you with a fait accompli. It costs more and is less readily available than generic spar varnish. For a serious wooden boat project maintained with natural materials, it is worth the difference.

What none of these products is, despite labels that sometimes imply otherwise, is a substitute for good wood selection, correct end grain sealing, and regular maintenance. A naturally durable species well treated with penetrating oils before a surface coat is applied will outlast any species treated only with a surface film, regardless of which film it is.

References: Bob Flexner, Understanding Wood Finishing, Fox Chapel Publishing — for alkyd and urethane resin chemistry. Le Tonkinois product documentation at letonkinois.fr for formulation details and application guidance.

VAKA designs are built for people who want boats that can be maintained rather than replaced. Plans at VAKA Boatplans; the full knowledge base at Field Notes.