What Are Microporous Coatings and When Should They Be Used on Timber?

Summary

Traditional alkyd gloss paint on exterior timber works as a film-forming coating — it sits on the surface of the timber and creates a physical barrier against water ingress. This works well when the film is intact and continuous. The problem is that timber breathes: moisture in the timber beneath the paint film changes with seasonal temperature and humidity cycling, and the vapour pressure generated inside the timber can exceed the adhesion of the paint film. When this happens, the paint blisters, cracks, and eventually peels — allowing water in at the failure point and accelerating the rot cycle.

Microporous coatings solve this by being vapour-permeable — they allow the moisture vapour to pass through the film while the surface tension of the micropore structure prevents liquid water from penetrating inward. The result is a coating that does not trap moisture below the surface, dramatically reducing the hydraulic pressure that causes conventional paint to delaminate.

The concept is well established and products have been available in the UK market since the 1980s. Scandinavian manufacturers (Sadolin, Sikkens, Osmo, Teknos) were early to market, driven by the demanding northern European climate. UK manufacturers including Dulux Trade and Johnstone's have developed their own ranges. However, the term "microporous" is used loosely in UK marketing and not all products labelled as such provide the same degree of vapour openness — checking the sd value (vapour diffusion resistance layer thickness, in metres) is the only reliable measure.

Key Facts

• sd value (equivalent air layer thickness) — The key measure of vapour permeability; lower sd = more vapour-open; typical microporous timber coating: sd 0.01–0.3m; standard alkyd gloss: sd 0.5–1.5m; barrier-type coatings: sd 5–20m+
• Sikkens Cetol products — Long-established microporous translucent stain system for exterior joinery and cladding; available in a range of colours and sheen levels
• Sadolin products — Sadolin Base (primer/preservative), Sadolin Classic and Sadolin Extra (translucent stains), Sadolin Superdec (opaque microporous paint); widely specified in UK trade work
• Osmo UV Protection Oil — Hardwax oil/microporous system for cladding; penetrating rather than film-forming; very low sd value; requires bare timber
• Dulux Trade Weathershield Exterior Gloss — Marketed as microporous; suitable for exterior joinery; check current datasheet for sd value confirmation
• Application over existing conventional gloss — Generally not recommended to apply microporous coating directly over alkyd gloss; the underlying non-permeable layer negates the benefit; ideally strip to bare timber or at minimum ensure all existing gloss has been removed where the timber is exposed
• Timber species considerations — High-tannin hardwoods (oak, teak, iroko) should be primed with aluminium primer before any coating; resinous softwood knots must be knotted with shellac before priming, even under microporous systems
• Application temperature — Most microporous coatings must be applied between 5°C and 30°C; avoid application in direct sunlight (skin-forming before penetration) or in frost
• Typical lifespan — Well-applied microporous systems on properly prepared joinery: 5–8 years between maintenance coats (compared to 2–4 years for conventional alkyd on poorly ventilated joinery)
• Maintenance cycle — When the finish coat shows weathering (chalking, slight loss of colour or sheen), a maintenance coat of the finish product applied without stripping back extends the life significantly; this is a key advantage over film-forming coatings that must be stripped on failure
• Opacity — Microporous coatings are available as opaque (solid colour, like paint) or translucent (like a stain, showing the wood grain); translucent products allow the natural wood to show through; opaque products are more appropriate for joinery in painted colour schemes

Quick Reference Table

Detailed Guidance

How Microporous Coatings Work

The term "microporous" refers to the microscopic structure of the dried paint film. Unlike a conventional film-forming coating that creates a dense continuous layer, a microporous coating contains millions of tiny pores (too small to allow liquid water droplet penetration — typically less than 10 nanometres) that allow water vapour molecules (much smaller than liquid water droplets) to pass through.

The physics is similar to waterproof-breathable fabric membranes in outdoor clothing (GORE-TEX and equivalents): liquid water is repelled by surface tension, but water vapour can diffuse through the micropore network. The sd value quantifies this in a practical way: an sd of 0.1m means the coating offers the same resistance to vapour diffusion as a 0.1m thick layer of still air.

For exterior joinery in the UK climate, the primary problem is moisture cycling within the timber (seasonal changes in MC between approximately 10% in summer and 18% in winter for exposed joinery). A film-forming coating traps this moisture as vapour pressure builds — micro-blister formation, adhesion failure, and delamination follow. A microporous coating allows the vapour to escape harmlessly.

Failure Modes of Standard Gloss on Timber

Understanding why conventional gloss fails on exterior timber helps explain when microporous products are the correct specification:

Blistering: Visible bubbles in the paint film, usually appearing on south-facing surfaces in summer. As the sun heats the timber, moisture vapour within the timber expands and pushes against the paint film. If the film is not vapour-open, it separates from the substrate. Once a blister forms, moisture gets under the film and rot follows.

Peeling: The paint film loses adhesion along a line, typically at a joint, grain line, or previously failed area. The underlying timber is often discoloured, damp, or shows early fungal staining. Peeling is the end stage of the blistering/moisture-trapping failure cycle.

Cracking at joints: Movement joints between timber sections (such as the junction between a window frame and casement) open seasonally. A rigid film-forming gloss cracks at these joints rather than flexing. Once cracked, water enters the joint, swells the wood, and the crack widens.

Microporous coatings are more flexible and vapour-permeable, reducing all three failure modes. However, they do not completely eliminate the need for maintenance — UV degradation, surface soiling, and mechanical damage still accumulate over time.

Compatibility with Existing Paint

The most common question with microporous coatings is whether they can be applied over existing conventional gloss paint. The answer depends on the condition and type of existing coating:

Ideal situation — bare timber: Apply a microporous primer or preservative base coat, then the microporous topcoat system. Maximum vapour openness, best adhesion, longest life.

Existing microporous coating in reasonable condition: Apply a maintenance coat of the same or compatible product after light sanding to remove surface degradation. No stripping required.

Existing alkyd gloss in poor condition (peeling, cracked): Strip completely to bare timber. There is no value in applying a microporous topcoat over a failing alkyd base — the failure mechanism (trapped moisture beneath the alkyd layer) continues regardless of the topcoat choice.

Existing alkyd gloss in sound condition: This is the grey area. Many microporous coating manufacturers advise against applying directly over alkyd gloss, as the alkyd layer beneath reduces the vapour permeability of the system. Some manufacturers allow overcoating of sound, lightly sanded alkyd gloss with their microporous topcoat. In practice, on a well-maintained joinery system with no moisture problems, overcoating with a microporous product is a viable option — but the fundamental vapour permeability benefit is reduced by the alkyd layer below. Inform the client of this limitation.

Timber Species Considerations

Different timber species behave differently under microporous coatings:

European redwood / Scots pine (most common UK window frames): High resin content; shellac knotting of all knots before priming is essential. Even microporous systems will not prevent resin bleed through an unprepared knot. Apply preservative primer (microporound or Sadolin Base) before topcoat.

Meranti / European hardwood joinery: Lower resin content; standard preparation applies. The grain tends to be more consistent than softwood.

Oak, iroko, teak: High tannin and oil content; standard primers have poor adhesion. Use aluminium wood primer as the first coat to seal tannins and oil, then proceed with the microporous topcoat system. Do not use standard white primer on high-tannin hardwoods.

Western red cedar (used in cladding): Highly stable, naturally durable, but releases tannin compounds that stain adjacent masonry and cause adhesion problems with some coatings. Use a specific cedar primer or aluminium primer before the topcoat system.

Modified timber (Accoya, Kebony, thermally modified): These engineered timbers have very low equilibrium moisture content and high dimensional stability. Standard microporous coatings are generally suitable, but check the manufacturer's specification for modified timber as some have additional requirements.

Application Method and Sequence

For a new timber joinery system using Sadolin or equivalent:

1. Check moisture content. MC should be below 18% (ideally 12–15%) before coating. Use a pin-type MC meter.
2. Sand. 80-grit to remove all mill glaze, followed by 120-grit for finishing. Sand in the direction of the grain.
3. Remove dust. Vacuum and wipe with a clean dry cloth.
4. Treat knots. Apply two coats of shellac knotting solution to all resinous knots. Allow each coat to dry (10–15 minutes).
5. Apply preservative/primer. Apply Sadolin Base, Sikkens Cetol Base, or equivalent. This penetrates into the timber and provides preservative protection. Two coats on end grain, one coat on face grain.
6. Apply topcoat. Apply two to three coats of the selected microporous finish (translucent or opaque). Allow the stated inter-coat drying time. Apply in thin, consistent coats — heavy application slows drying and can cause runs.
7. End grain treatment. Pay particular attention to end grain sections (joints, sills) — these are the primary entry points for moisture and must receive liberal coats of preservative base coat.

Frequently Asked Questions

Why does my timber window frame keep peeling even after stripping and repainting?

The most likely causes are: (1) the existing system is a film-forming alkyd and moisture cycling is generating vapour pressure; switch to a microporous system; (2) the MC of the timber was too high when painted (above 18%); allow the timber to dry thoroughly before coating; (3) the end grain joints were not adequately primed with a preservative base; treat end grain generously; (4) there is active rot in the frame that needs to be cut out before repainting.

Do microporous paints need a special primer?

Yes, typically. Most microporous coating systems are designed around their own primer/base coat product (e.g. Sadolin Base, Sikkens Rubbol Primer). These primers are formulated with preservative additives and compatible chemistry to achieve the best adhesion and vapour permeability. Using a standard alkyd primer under a microporous topcoat partially defeats the purpose.

How often do microporous systems need to be recoated?

A well-applied system with thorough initial preparation and good exposure position (not permanently shaded or permanently exposed to driving rain) typically needs a maintenance coat every 5–8 years. The maintenance coat does not require stripping — sand lightly to remove chalking and surface degradation, then apply the topcoat only. This is significantly less work than the full strip-and-repaint cycle required by failed conventional gloss.

Are microporous coatings suitable for fascias and soffits?

Yes. Fascias and soffits benefit from microporous coatings in the same way as windows and doors — moisture cycling in the timber causes conventional paint to fail at the fixings and at board joints. Apply a preservative base coat and a vapour-open topcoat for best longevity. For uPVC fascias that have been overfaced, use a UPVC-compatible microporous paint (or simply use a UPVC paint — microporous properties are less relevant on non-timber substrates).

Regulations & Standards

• BS 6150 — Painting of buildings: code of practice; guidance on primer and topcoat selection for exterior timber; references vapour-open coatings in relevant sections [verify current edition]

• BS EN 927-1 — Paints and varnishes: coating materials and coating systems for exterior wood; classification and selection

• BS EN 927-2 — Test methods for vapour permeability (sd value); measurement standard for vapour diffusion resistance

• Sadolin UK — Superdec and microporous coating guidance — Product data, system specifications, and application guides

• Sikkens (Akzo Nobel) — Cetol system technical data — sd values, application guidance for Cetol range

• Dulux Trade — Exterior wood coating specification — Weathershield and microporous exterior product data

• BWFA (British Wood Finishing Association) — Industry guidance on exterior timber coatings

• Historic England — Exterior joinery painting guidance — Context for microporous coatings on historic buildings

• exterior paint preparation — Substrate preparation for exterior timber

• paint types guide — General alkyd vs acrylic comparison

• specialist coatings — High-performance coatings for structural applications