Collection: Field Notes — Preserving Natural Materials at Sea 

Series Hub: Preserving Wood 

Why choosing between these two wood finishes is less a technical decision than a philosophical one — and why the philosophy matters

I want to be honest about where this investigation started. I came to linseed oil not through rigorous comparison but through instinct — the instinct that a boat built from natural materials ought not to be sealed under a skin of synthetic polymer. That instinct may be right. It may be sentimental. The only way to find out is to test it properly, follow the old sources, and report what actually happens rather than what I expected.

This is that report, so far. It is not finished. It probably will never be finished. Wood finishing is one of those territories where the more carefully you look, the more the certainties dissolve into conditions and variables. What I can offer is what I have found, where I found it, and what I still don't know.

The Real Question Underneath the Question of Linseed Oil against Polyurethane

Linseed oil and polyurethane are not simply two different ways of achieving the same end. They represent different ideas about what a finish is for — and those ideas produce different relationships with the wood, with maintenance, and eventually with the water the boat lives on.

Polyurethane builds a film above the wood surface. Linseed oil becomes part of the wood itself. That difference in where the finish lives determines almost everything that follows — how the wood moves under it, how water behaves at the boundary, how the finish fails when it fails, and what you have to do about it.

I did not understand this when I started building. I understood it the way you understand something you have read. The difference between that and understanding it as a practical matter — as something that has consequences for a boat you are responsible for — took time and some failures to establish.

How Each Material Works

Linseed is a drying oil. A small number of plant oils share this property — tung is the other one worth knowing — defined by their ability to cure not by evaporation but by chemical reaction with atmospheric oxygen. The fatty acids in linseed oil, particularly alpha-linolenic acid, carry multiple double bonds that oxygen attacks, forming cross-links between adjacent molecules until the oil solidifies throughout its depth. The finish does not sit on the wood. It hardens within it.

One thing the old sources are unambiguous about: raw linseed oil is not the material for this. Ashmun Kelly's The Expert Wood Finisher — a book I return to regularly and recommend without reservation — states it plainly: raw oil dries soft, remains tacky, and produces nothing durable. The material requires metallic driers to make curing practical. What results is boiled linseed oil, and when I say linseed oil anywhere in these notes, that is what I mean. There is a further option worth noting: heat-polymerised linseed oil, made by cooking raw oil to around 300°C until it begins to thicken, produces a faster-curing finish without metallic driers — better for those concerned about the environmental impact of cobalt and manganese compounds in conventional boiled oil. I have not yet made my own batch but intend to. That experiment will get its own notes.

Polyurethane works differently at every level. It is a synthetic polymer in a solvent carrier, curing as the solvent evaporates and the resin cross-links into a continuous film above the wood surface. Bob Flexner's Understanding Wood Finishing — the other book I keep within reach — is precise about what this hardness means in practice: polyurethane is the most abrasion-resistant of the common varnishes, which is exactly why it suits interior floors and dining tables, and exactly why it fails on exterior wood. Hardness and rigidity are the same property. Rigidity is the enemy of any coating on wood that moves.

Where Polyurethane Earns Its Place

Before making the case against it on exterior and marine wood, I want to be fair. Polyurethane is not a bad material. It is the wrong material for a specific application, which is different.

For hardwood floors under daily heavy use — chairs dragged, dropped objects, wet boots — oil finishes are inadequate on their own. A linseed-oiled floor under that kind of traffic wears through quickly. The same floor finished with oil-based polyurethane holds up for years. The hardness that makes it unsuitable for outdoor wood is a genuine virtue on a horizontal interior surface taking mechanical punishment.

For figured wood where visual depth and gloss are the priority — curly maple, quartersawn oak, any timber with interesting figure — polyurethane builds a film that enhances those qualities in ways that a penetrating oil finish, which essentially disappears into the surface, cannot match. If the appearance of deep, clear finish is the goal, urethane systems achieve it more readily. Le Tonkinois sits interestingly between the two — a tung oil and resin compound that builds more than pure oil but remains more flexible than urethane. I have used it and found it worth knowing about.

Where the Trouble Starts

Wood moves. It expands across the grain as moisture rises and contracts as it dries, continuously and significantly in any outdoor or marine situation. A rigid film on a moving substrate has only one available response: it cracks. Flexner confirms this failure mode directly. He notes that spar varnish — which carries far more oil than standard varnish and cures to a much more flexible film — is the appropriate film finish for exterior wood precisely because flexibility rather than hardness is what exterior wood needs. Polyurethane is the wrong specification for the application. This is a technical judgement, not a preference.

Once the film cracks, the damage becomes structural in a way that is not immediately visible. Water enters the crack, gets behind the film, and is then effectively sealed in — the intact surrounding film prevents it from evaporating. You have created a persistently moist environment in exactly the location where rot needs sustained moisture to establish. The surface looks sound from any distance. The rot below it does not announce itself until the damage is significant.

This is the standard failure mode of polyurethane on exterior and marine wood. It is reliable and it is not slow.

How Linseed Oil Fails Differently

There is no film to crack. What happens instead is gradual depletion: the polymerised oil oxidises further under ultraviolet light, becomes brittle at the outermost surface, and the wood begins to grey and check. This is the material telling you something before anything structural is at risk. The wood is readable in a way that polyurethane-coated wood is not.

Maintenance is a fresh coat of oil applied to a clean surface — no stripping, no sanding to key the surface, no preparation beyond removing dirt and allowing the wood to dry. The new oil integrates with what remains of the previous treatment. The maintenance culture that linseed oil requires is the opposite of the one that polyurethane requires, and that difference compounds over years.

A Danish condition survey of ten linseed oil-treated buildings, published in 2002 by E. Brandt and T. Lading, produced a finding I have thought about often: across all surveyed examples, there were no instances of wood in poor condition beneath an essentially intact linseed oil coating. The authors contrast this explicitly with latex paint, where rot behind intact-looking film is a documented and common problem. The material that lets water in also lets it out. The wood beneath it stays accessible to air.

What Linseed Oil Does Not Do

This is where I want to be careful, because the case for linseed oil is often overstated and those overclaims are easy to challenge.

Flexner states plainly that of all common finishes except wax, boiled linseed oil is the least protective in terms of surface hardness, scratch resistance, and stain resistance. Liquid water works through a brush-applied linseed oil coat and can raise the grain within minutes. Water vapour passes through almost as if the finish were not there. If protection means a hard barrier against mechanical damage and liquid contact, polyurethane wins comprehensively and it is dishonest to claim otherwise.

The honest case for linseed oil rests on different ground. Research by Miha Humar and Bostjan Lesar at the University of Ljubljana, published in International Biodeterioration and Biodegradation in 2013, tested linseed and tung oil treated timber against wood-decay fungi and measured water uptake under laboratory and outdoor conditions. They found that oil treatment measurably reduced water uptake and provided genuine fungal resistance — fungal mass losses in linseed-treated timber were roughly half those in untreated controls. Critically, they found that one of the most important advantages of oil-based treatments is that they do not seal the surface, and therefore allow the wood to dry after precipitation. Reducing average moisture content over time is sufficient to slow fungal attack significantly, even where the oil does not prevent water from entering entirely.

The caveats matter. The Humar study used vacuum-pressure impregnation at high oil retentions — far higher than brush application achieves. The benefits scale with penetration depth. For brush-applied coats on a frame, the fungal resistance and moisture reduction are real but modest compared to deeply impregnated specimens. The oil slows moisture cycling. It does not prevent it.

Tung oil, for what it is worth, outperformed linseed oil on both water exclusion and fungal resistance throughout that study. I have not yet run my own comparative test between the two under field conditions. That is on the list.

The Application — Where Results Are Made or Lost

The Brandt and Lading survey found highly variable results across their ten buildings — from failure within two years to excellent condition after eight or more years — with workmanship and application conditions accounting for most of the difference. This is the part of the field notes that most published sources understate.

The wood must be genuinely dry at application. Below 12–15% moisture content. Brandt and Lading identify this as the single most important factor for a good result: damp wood cannot be properly saturated and the treatment fails from the start regardless of subsequent care.

Temperature matters. Below around 5–10 degrees Celsius, boiled linseed oil becomes difficult to apply in thin coats and drying times extend significantly. Apply in a heated space or wait for appropriate weather. In my experience, a cold February workshop produces unpredictable results. A mild October day in a shed with a small heater running does not.

Inter-coat drying intervals are longer than almost every guide suggests. The most durable treatment in the Danish survey used five coats at one-week intervals — more than six weeks in total before the surface was considered complete. Rushing this produces thick, incompletely cured layers that wrinkle and crack rather than building into a sound treatment. Apply thin, wipe back all excess before the coat becomes tacky, allow each coat to cure fully. The patience required is the part that is hardest to pass on to someone in a hurry to get their boat in the water.

The Environmental Question

The boats I build are made from materials that compost when they are finished with. The finish they carry ought to be consistent with that. Polyurethane weathers into synthetic polymer fragments — sanded polyurethane produces microplastic particles directly; weathered coatings fragment progressively over their service life and enter the water the boat lives on. Boiled linseed oil degrades into organic fatty acid compounds that break down in the environment rather than accumulating in it.

This is not a peripheral concern dressed up as technical reasoning. It is part of the same question about what a boat needs to be and what it leaves behind. The fibreglass disposal problem is the extreme version of this argument. Synthetic coatings on wooden boats are a quieter version of the same story.

Where This Investigation Is Going

I have not yet run a controlled comparison of brush-applied linseed oil against polyurethane on matched timber samples under outdoor conditions. I have used both materials, on different projects, over several years. The qualitative difference in maintenance behaviour is as the sources describe. The quantitative difference in moisture uptake under field conditions — brush-applied, real-world temperatures, actual marine exposure — is something I want to test properly and document here.

I also want to test heat-polymerised oil against conventional boiled linseed oil under the same conditions. And tung oil against both. The published research gives a starting point. What it does not give is results from a small workshop on the Essex coast, in the conditions that the people reading these notes are actually working in.

That is what the Field Notes is for. The experiments are ongoing. The Compendium will carry the conclusions when enough of them have held up long enough to be worth calling conclusions.

Until then: this is where I am.

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).

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