Does it change the colour of the wood?

The product is not a stain. On some species a treated surface can look slightly darker once dry. Test a discreet area, that is the only way to know on your wood.

Can I treat wood that has already gone grey?

You can, but the grey stays: it is an altered surface layer. If the look bothers you, sand first, let it dry, then protect the fresh wood.

How much product does a fence take?

Count both sides, which plenty of people forget. One 19 L pail covers 1,000 to 1,200 sq ft in two coats, and new or sanded wood pulls toward the low end of that range.

Protecting exterior wood: deck, fence, siding

Short answer

Exterior wood greys because UV degrades the lignin at the surface, not because it is dirty. A water repellent reduces absorbed water and slows the wear, but it does not colour the wood. If you want colour, stain first, let it dry completely, then protect.

Why does exterior wood grey?

Solar radiation degrades lignin, one of wood’s components, within the first fractions of a millimetre of the surface. The literature on wood photodegradation documents this, and US Forest Service work shows that even violet and visible light contributes, not only UV in the strict sense.

So the grey you see is not dirt. It is an altered surface layer, which rain eventually carries off, leaving the wood rougher.

What does water add?

Wood is hygroscopic: it takes up and releases moisture, and it swells and shrinks accordingly. That movement opens checks, and checks hold more water.

In Quebec the cycle ends in a freeze. Water trapped in a check freezes, gains volume and widens the check. That is why an exposed deck degrades faster here than in a mild climate.

What does a water repellent change on wood?

It reduces how much water the wood takes up, and therefore the size of the swell-and-shrink movement, and it resists UV, which slows surface degradation.

What it does not do: it does not stain, it does not fill an existing check, and it does not replace sanding on wood already grey. New or freshly sanded wood absorbs markedly more than wood already treated, so plan for the low end of the coverage range.

Stain or repellent, in what order?

The two are not opposed, they do different jobs. A stain brings colour and pigments that filter part of the radiation. A penetrating repellent handles water.

If you want both: stain first, let it dry completely, then apply the protection. And always test a low-visibility board before treating the whole deck, because the result depends on the species and the condition of the wood.

Frequently asked questions

Does it change the colour of the wood?: The product is not a stain. On some species a treated surface can look slightly darker once dry. Test a discreet area, that is the only way to know on your wood.
Can I treat wood that has already gone grey?: You can, but the grey stays: it is an altered surface layer. If the look bothers you, sand first, let it dry, then protect the fresh wood.
How much product does a fence take?: Count both sides, which plenty of people forget. One 19 L pail covers 1,000 to 1,200 sq ft in two coats, and new or sanded wood pulls toward the low end of that range.

Sources

These are the published sources behind the claims above. They describe the product CLASS (silane and siloxane repellents), not tests on Nanoxeal itself. We keep that distinction.

The science behind Nanoxeal →