Staining and Sealing a Wood Fence in Montana's Climate

Do you have to stain a cedar fence?

No, but the wood changes either way. The Forest Products Laboratory's chapter on finishing explains that unfinished wood outdoors changes color, is roughened by sunlight and surface checking, and slowly erodes. The ultraviolet light in sunlight breaks down lignin, the natural adhesive between wood fibers, so loose fibers wash away. That erosion is slow, on average about a quarter inch of wood per century, so a bare fence board does not wear through. What it does is gray, check and warp.

The handbook notes that the gray color is largely mildew and other microorganisms on the weathered surface, not just sun. Weathering also makes boards cup and loosens fasteners, and cupping is worse in wide, flat-grain boards; for best resistance, it says a board's width should not exceed eight times its thickness. So a natural gray fence is a legitimate choice, but it is not a maintenance-free one.

Does pressure-treated or naturally durable wood still need a finish?

For looks and stability, yes. The handbook says to use pressure-treated or naturally durable wood for all posts and other fence parts in ground contact, and that above ground the service life of the two is quite comparable. In above-ground uses, weathering can be as much a factor in long-term life as decay resistance. Whichever you use, it says the wood needs protection with a finish, and periodic treatment with a penetrating sealer such as a water-repellent preservative reduces checking and splitting.

It also notes that the common preservative treatments do not significantly change how wood weathers, so treated wood still checks and cracks in the sun, and that waterborne-treated wood can be finished once it is properly dried and clean. Oil-borne treatments such as creosote and pentachlorophenol are not considered paintable. Our comparison of wood, vinyl, metal and composite fences covers why many owners pick a material for its maintenance as much as its looks.

Which finish is best for a fence?

For most wood fences, a penetrating finish. The handbook groups exterior finishes into penetrating finishes (water-repellent preservatives and semitransparent stains) and film-forming finishes (solid-color stains and paint). Penetrating finishes soak in and wear away gradually, so recoating is a matter of cleaning and brushing on another coat. Film-forming finishes protect more but eventually peel or crack and need scraping and sanding before recoating.

Texture matters. The handbook says semitransparent stains give much better service life on rough-sawn wood than on smooth wood, and that even paint and solid stains last longer on roughened surfaces. It also says paints and stains based on acrylic polymers resist UV better than those based on oil. Solid-color stains behave much like paint and are not generally recommended for horizontal surfaces, so keep them off rail tops and caps if you use them on the boards.

A second Forest Products Laboratory report, written about exterior wood in the humid South but with fence advice that carries over, is more direct. It says many fences are simply left to weather, but if a finish is wanted, semitransparent penetrating stains or water-repellent preservatives with a mildewcide are preferred because they soak in without forming a film, so they do not crack or peel. It does not recommend varnish or solid-color stain for exterior fences, because they will not withstand sun and rain. It also advises treating both treated and untreated fence wood with a water repellent or water-repellent preservative, liberally at board ends and wherever pieces touch.

How long does fence stain last?

It depends on the surface and the number of coats. According to the handbook, one coat of semitransparent stain on new smooth wood is expected to last only 2 to 4 years, while a two-coat system on rough wood may last as long as 10 years in certain exposures, because so much stain is absorbed. Successive recoats last longer than the first, because the stain penetrates the small checks that open as wood weathers.

Exposure changes everything along a single fence. A south or west-facing run in full sun weathers faster than a shaded north side, and the bottom boards where snow sits and sprinklers hit stay wet longest. Expect to recoat the hard-working sections first rather than the whole fence on one schedule. Our guide to snow and frost damage on fences covers the moisture side of that.

How should stain be applied to a fence?

The handbook's method for oil-based semitransparent stain on rough or weathered wood is two coats applied wet on wet. Stain a few boards or one panel at a time to avoid lap marks, let the first coat soak in for 20 to 60 minutes while you work elsewhere, then apply the second before the first dries; if the first dries completely it can seal the surface so the second cannot penetrate. About an hour later, wipe off stain that has not soaked in, or it can leave a shiny film. Back-brush whether you brush, roll or spray, work in shade where you can, and do not mix brands or batches.

Waterborne semitransparent stains do not penetrate as well as oil-borne ones, the handbook says, but they are easier to apply and less likely to lap; a second coat improves durability and can go on any time after the first dries. One safety point it stresses: rags and sponges wet with oil-based stain can spontaneously combust, so put them in water in a sealed metal container right after use.

When is the right time to stain a fence in Montana?

Temperature is the main limit here. The handbook says latex-based waterborne paints should be applied at 50 degrees Fahrenheit or warmer, and that for proper curing the temperature should not drop below 50 degrees for at least 24 hours after application; oil-based paint can go on at 40 degrees or warmer. Read the label of the product you use, but in western Montana that usually rules out cold shoulder-season days and nights, so plan staining for late spring through early fall, on dry days with dry wood.

New wood should not wait too long either. The handbook says the paint bond is greatly improved if wood is painted within a week of installation, weather permitting, because even short weathering degrades the surface for paint. For stains on new lumber, and especially on treated wood that needs to dry after treatment, follow the product label for timing.

Why do the ends and bottoms of boards fail first?

Because end grain drinks water. The handbook explains that end grain absorbs moisture much faster than face grain and finishes generally fail there first. It recommends applying a water-repellent preservative or primer to end grain on fences and railings fully exposed to the weather, and finishing all surfaces, since a finish on only one side causes uneven moisture uptake and cupping. On a fence that means picket tops, board bottoms, rail ends and post tops.

Design helps the finish. Boards held a little off the ground, post caps, and rails that shed water all keep end grain drier. Those are construction details worth discussing before a custom fencing project starts, and they matter as much for longevity as how deep the posts go; see frost depth and fence post footings.

How do you know when a fence needs recoating?

The handbook's test for a water-repellent finish is simple: splash some water on the wood. If it beads and runs off, the treatment is still working; if it soaks in, it is time to refinish. Graying is another signal. To recoat a semitransparent stain, brush off dirt, dust and loose fibers with a stiff dry brush and apply a new coat; heavy scraping and sanding are not usually needed. Solid stains and paint need loose material scraped, bare spots sanded and the surface cleaned first.

What causes black streaks and dark spots on fence boards?

Usually mildew or iron. The handbook's test for mildew is a drop or two of fresh household bleach on the dark spot: mildew usually bleaches out in a minute or two, and dirt does not. Mildew must be killed with a mildew cleaner or a dilute bleach and detergent wash before refinishing, or it grows through the new finish, and bleach must never be mixed with anything containing ammonia.

Dark blue-black stains around nails are iron reacting with tannins in the wood, and reddish streaks are rust. The handbook says both are prevented by corrosion-resistant fasteners such as high-quality galvanized, stainless steel or aluminum, and that iron stain can be removed with oxalic acid, used carefully because it is toxic, but will return if the iron source remains. It is worth asking any installer what fasteners they use.

How HJ can help

We build and repair wood fences across Missoula and the Bitterroot and can advise on finish choice, timing and the construction details that help a finish last. If boards are cupped, posts are leaning, or a gate drags, see fence repair and why gates sag. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free fence estimate and we will walk the line with you.

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