Deck and Garage Floor Coatings: Preparation Is Most of the Job

Coatings on horizontal surfaces have a harder life than anything on a wall. They take foot traffic, standing water, tyre heat, dropped tools and ultraviolet light, and they are the finishes people are most likely to describe as having failed. In practice they very rarely fail as a film. They fail at the bond between the coating and the substrate, and that bond is decided before the first coat goes on.

Concrete: the moisture test

Concrete looks dry long before it is dry, and moisture rising through a slab will push a coating off from underneath regardless of how well it was applied. Before coating any slab, particularly one at or below ground level, tape a square of plastic sheet tightly to the surface on all four sides and leave it for at least sixteen hours.

If there is condensation under the plastic or the concrete beneath it has darkened, the slab is passing moisture and needs either a moisture-tolerant system or a vapour barrier. Coating it anyway will produce blisters within months.

Concrete: profile and contamination

New concrete is usually sealed at the surface by the finishing process and will reject a coating. It needs etching or mechanical grinding to open the pores — the target is a texture roughly like fine sandpaper. Older slabs need degreasing first, because oil in a garage floor goes deeper than the stain suggests and no coating bonds to it.

Test by dripping water on the surface. If it beads, the slab is either sealed or contaminated and is not ready. If it soaks in within a few seconds, the profile is right.

Timber decks: cleaning and moisture

A deck needs to be clean, dry and free of any previous film-forming coating that is peeling. Cleaning means removing grey weathered fibres and mill glaze, not just dirt; a deck cleaner and a stiff brush, or a careful pressure wash, will do it.

Pressure washing deserves a warning. Held too close or at too high a pressure, a washer erodes the soft wood between the grain and leaves a furry, damaged surface that drinks stain unevenly and looks worse than what you started with. Keep the fan wide, keep moving, and stay well back.

After washing, a deck needs days to dry, not hours. Most stains specify a maximum moisture content, and applying to damp timber traps water under the finish.

New timber and mill glaze

New pressure-treated decking is often too wet to coat and has a compressed, closed surface from the milling process. The usual advice is to wait — sometimes several months — until the boards accept water rather than beading it. A light sanding also opens the surface.

Coating new decking too early is one of the most common causes of a stain that sits on the surface, wears off in patches and is blamed on the product.

Film-forming versus penetrating

Penetrating stains soak into timber and wear gradually, fading rather than peeling. They are easy to maintain — clean and recoat — and offer less protection against water and abrasion.

Film-forming coatings sit on the surface, protect much better and fail dramatically when they fail, because water gets underneath and lifts a sheet. On a horizontal surface exposed to weather, that trade-off is real and worth thinking about before committing, because moving from a film to a penetrating product later means stripping.

Temperature, and the trap of a warm afternoon

Every coating states a temperature range, and horizontal surfaces routinely sit well outside it even on a mild day. A concrete slab or a dark deck board in direct sun can be thirty degrees hotter than the air. A coating applied to a hot surface flashes off too quickly, does not flow, and does not wet the substrate properly.

Work in the shade or in the cooler part of the day, check the surface temperature rather than the air temperature, and stop before evening dew if the product is water-based.

Before you open the tin

• Moisture test on any slab.
• Water-drop test for profile and contamination.
• Deck fully dry after cleaning — days, not hours.
• Surface temperature within the stated range, in shade.
• Enough product for the whole area in one session.
• A plan for edges, so you are not standing on wet coating.

Coverage and cure data for everything we carry is on the product pages in deck and floor coatings, and the cleaning and priming products are in primers and sealers.

Test patches are not optional

Before coating a whole deck or slab, do a test area of a few square feet in a corner and let it cure fully. Timber in particular takes stain differently depending on species, age and how much of the mill glaze remains, and a colour that looked right on a card can be markedly different on your boards.

A test patch also reveals adhesion problems early. If the coating can be lifted with tape after curing, the preparation was not sufficient and you have learned that on four square feet rather than four hundred.

Maintenance intervals

Horizontal coatings are consumable. A penetrating deck stain typically needs refreshing every one to three years depending on exposure; a film-forming coating lasts longer and then requires far more work when it fails.

The cheapest maintenance is annual cleaning. Removing dirt, algae and debris before it works into the surface extends the life of any coating considerably, and a deck washed each spring will hold a finish far longer than one left to weather and then stripped every few years.