Sadolin Superdec vs Cuprinol Garden Shades on a 15 Square Metre Larch Cladding Run

Larch has a way of exposing weak assumptions about exterior coatings. It is oily, it moves through every wet-dry cycle, and bare boards can silver to grey within eighteen months. Sadolin Superdec and Cuprinol Garden Shades deal with that movement and greying in very different ways. Superdec is a self-priming opaque woodstain that dries to a satin skin. Garden Shades is a matt, low-build colourwash that sits closer to the fibre. Across a 15 m2 run of vertical board, the difference shows up quickly.

Litres on sawn boards

Cuprinol quotes Garden Shades at roughly 6 to 8 square metres per litre per coat on smooth planed timber. Sawn or rough-sawn larch takes more product, so 4 to 5 m2 per litre is a more realistic allowance. Two coats across 15 m2 of sawn board can use 6 to 7 litres.

Superdec quotes about 12 to 16 m2 per litre per coat on planed wood. On larch, the first coat often behaves like a primer coat because the board drinks it, so half the stated figure is a safer assumption for coat one.

At 4.5 m2 per litre, Garden Shades needs close to 6.7 litres for two coats over 15 m2, which means roughly three 2.5 litre tins. Superdec, allowed at 8 m2 per litre for the first coat and 14 m2 per litre for the second, comes out nearer 2.9 litres in total. That is one 2.5 litre tin plus a small top-up. The opaque product costs more per litre, yet the rough-board coverage can make it the cheaper material bill.

Greying and the way each finish fails

Bare larch greys because ultraviolet light breaks down lignin at the surface, then rain washes away the degraded fibre. Garden Shades slows the process without stopping it. Its low film build lets some UV through and allows moisture to pass both ways. That breathability is also why it will not peel: there is almost no skin to lift.

Superdec builds a true opaque film and blocks UV outright. Its colour holds for longer because the timber surface is hidden from the sun. When that film eventually gives way, failure usually begins as flaking at a board edge before it resembles even fading.

Around year six, the two products tend to ask for different repairs. A Garden Shades finish fades back toward the timber and can be refreshed with a wash and a fresh coat, with no sanding and no stripping. A Superdec finish keeps its colour for longer, although the first flake has to be caught before water tracks behind the film. On south-facing cladding, where heat and movement are strongest, that edge-flaking risk on larch deserves respect.

Moisture and surface prep

Neither coating tolerates damp or dusty boards. Aim for moisture content under 18 percent, brush dust out of the grain, and sand planed larch lightly at 120 grit to break any mill glaze. After recent rain, leave the boards two or three dry days before the first coat.

Brush work on vertical cladding

Vertical cladding shows every join. The same approach used on an indoor feature wall, cutting a crisp edge and rolling into it while wet, works on a board run outside.

Work one full board from top to bottom before moving across to the next. Keep the wet edge alive for the whole length. If a 3 metre board is stopped halfway down and finished later, the overlap dries as a visible band.

That band is especially stubborn with Garden Shades. A matt finish does not level and reflect light like a satin coating, so the joint can remain visible even after the surface has dried fully.

A 50 mm synthetic brush fits the tongue-and-groove shadow lines. A wider block brush or a short-pile roller covers the board face more quickly.

Superdec self-levels enough to hide most brush strokes if it is laid off in one direction. On a warm day, though, its skin time can leave only about ten minutes to keep an edge live.

Garden Shades stays open longer and forgives a slower hand. That makes it easier for a first-time coater working across a large elevation.

Use a kettle instead of working straight from the tin. Product that has thickened around the rim then stays out of the middle of a board.

End grain needs heavier treatment than the face. Board tops and cut ends are where water gets in, so flood them with an extra pass whichever product is used. Larch end grain wicks moisture like a straw, and one thin coat there is a common reason a good-looking job rots from the top down.

When the Osmo look is the real brief

Some buyers looking at larch are chasing the low-sheen, grain-forward look that Osmo Polyx oil gives on indoor stairs. Garden Shades gets closer to that exterior aesthetic because it stays matt and leaves the figure readable. Superdec reads more like a painted board, with colour first and grain second.

For many buyers, that visual split matters more than the durability figures. If larch was chosen because the timber itself should remain visible, an opaque woodstain works against that choice. If the aim was a warm-toned, low-maintenance elevation in a fixed colour that will not drift, the opaque film is the product that delivers the look.

Price, drying and the longer bill

Garden Shades in a 2.5 litre tin runs around GBP 22 to 28. Three tins for two coats on sawn larch puts the material cost around GBP 70 to 84. Superdec in 2.5 litre sits nearer GBP 40 to 48, and a 15 m2 job needs one tin plus a 1 litre top-up, so the material cost lands around GBP 55 to 65. On raw coating cost, Superdec can be cheaper on sawn timber even with the higher shelf price, purely because it spreads further.

Cuprinol Garden Shades touch-dries in an hour or two and can be recoated after four. If the weather holds, two coats can fit into one dry day. Superdec usually needs a longer gap between coats, typically overnight. Its thirsty first coat on larch can also tempt a third pass to even out the colour, which turns the job into a weekend task.

On exposed larch, Garden Shades typically wants refreshing every two to four years, depending on aspect. Superdec, protected as an opaque film, commonly holds five to eight years before it needs attention. The work is heavier when it does: cut back flaking, sand, spot-prime, then recoat.

Across a fifteen-year horizon, the two finishes can sit closer together on total cost than the first tin price suggests. Garden Shades asks for lighter, more frequent refreshes; Superdec gives a longer run before a more involved repair.

The board edge is the tell on these two finishes: a matt wash normally thins across the face, while an opaque film shows its weakness where water can get behind the skin.