How Wood Coatings Are Categorized
Finishes for outdoor wood fall into two broad categories based on how they interact with the wood surface: penetrating finishes and film-forming finishes. A third category — clear sealers — spans both depending on the formulation. Understanding the difference matters because the category determines how the finish fails, how it should be maintained, and what preparation is required before reapplication.
Penetrating Finishes
Penetrating finishes — including oil-based stains and penetrating oil-alkyds — absorb into the wood fibres rather than sitting on top of the surface. They reinforce the wood from within, leaving little or no film on the surface.
How They Behave
Because penetrating finishes are absorbed into the wood, they move with the wood as it expands and contracts seasonally. This makes them suitable for climates with significant moisture variation and temperature swings, including most of Canada. They do not crack or peel in the way film-forming coatings can, and they are generally easier to reapply since there is no failing film to strip before a new coat is added.
The trade-off is that penetrating finishes provide less surface protection against abrasion and standing water than film-forming options. On high-traffic horizontal surfaces like deck boards, they typically require reapplication every one to two years.
Common Formulations
Oil-based penetrating stains traditionally used linseed or tung oil as their base, sometimes combined with alkyd resins. Water-based penetrating stains use acrylic or other polymer binders and are lower in volatile organic compounds (VOCs). Many Canadian homeowners have shifted toward water-based options due to lower odour, easier cleanup, and faster dry times.
Film-Forming Finishes
Film-forming finishes build a continuous layer on the wood surface. Solid-colour deck paints and solid-body stains are the most common examples. Semi-transparent and opaque alkyd stains also form a partial film while allowing some penetration.
How They Behave
Film-forming coatings provide a durable, abrasion-resistant surface and can cover more wood defects and discolouration than penetrating stains. However, because the coating sits on top of the wood rather than inside it, it cannot expand and contract with the wood at the same rate. Over time — and particularly through freeze-thaw cycles — the coating begins to crack and peel.
Once a film-forming finish starts to fail, reapplication requires stripping the old coating. Applying a new film coat over a failing one results in accelerated failure of both layers. This makes the long-term maintenance cycle more labour-intensive than penetrating finishes.
When Film-Forming Coatings Are Appropriate
Film-forming coatings are often selected when the wood surface has significant colour variation or grey weathering that cannot be corrected with lighter stains. Solid deck paint also provides a consistent, uniform appearance that some homeowners prefer. On vertical surfaces like deck railings and fascia boards, film-forming coatings hold up better than on horizontal deck boards because they are not subject to standing water or foot traffic.
Clear Sealers
Clear sealers are designed to repel water without altering the wood's natural colour. They range from water repellents with minimal resin content to penetrating sealers with alkyd or acrylic binders that provide more durable protection.
On new cedar or redwood, a clear sealer can preserve the natural colour for one to two years before UV exposure begins to grey the surface. On pressure-treated lumber, a clear water repellent applied after the wood has dried can reduce checking and splitting during the first seasons of weathering.
Clear sealers alone do not provide UV protection unless they contain a UV stabiliser or pigment. Without UV protection, even a well-sealed deck will grey within a single season in most Canadian locations.
Applicability to Common Canadian Deck Woods
Pressure-Treated Lumber
Most Canadian decks use pressure-treated dimensional lumber (typically SPF — spruce, pine, fir) treated with alkaline copper quaternary (ACQ) or copper azole. Freshly treated lumber needs time to dry before accepting most oil-based finishes — this is commonly described as a period of several weeks to a few months, though the actual time depends on the lumber's moisture content at purchase.
Water-based acrylic stains can be applied sooner to pressure-treated wood than oil-based products, as they are less sensitive to residual moisture. Technical data sheets from individual manufacturers specify the recommended moisture content threshold.
Western Red Cedar
Cedar is naturally resistant to rot and insect damage due to compounds present in the heartwood. However, the surface still weathers and requires protection from UV and moisture. Cedar accepts both penetrating oils and semi-transparent stains well, and the wood's natural colour and grain are often worth preserving, making semi-transparent or clear products the common choice.
Factors Specific to the Canadian Climate
Two conditions in particular affect finish selection for Canadian decks:
Freeze-thaw cycling: In most Canadian cities, outdoor temperatures cross the freezing point multiple times between October and April. Water that enters small surface checks or grain openings freezes and expands, widening those openings. A penetrating finish reduces moisture uptake; a film that has already begun to fail accelerates this process by trapping moisture beneath the coating.
UV exposure: UV radiation degrades wood lignin, which is the binding agent that holds wood fibres together. Greying is the visible result, but the more significant issue is surface fibre erosion over time. Any stain or sealer that contains pigment or UV absorbers will slow this process. Clear products without UV protection do not.
Summary
For most horizontal deck surfaces in Canada, penetrating stains — particularly water-based semi-transparent formulations — offer a practical maintenance cycle: they do not peel, reapplication requires only cleaning and light prep rather than stripping, and they move with the wood through seasonal changes. Film-forming coatings are better suited to vertical surfaces or situations where colour uniformity is the primary concern. Clear sealers extend the life of any coating system when applied correctly but should not be the sole finish on unprotected wood.