Why a clear finish on cedar weatherboards fails so quickly

A clear finish fails quickly on cedar weatherboards because it lets ultraviolet light through to the timber, and the sun breaks down the surface of the wood underneath the coating. Once that surface layer degrades, the clear coat has nothing sound to hold onto, so it cracks, greys and peels, however good the product was.

That is why Resene's technical literature states plainly that it does not recommend clear coatings for exterior timber. What protects cedar outside is pigment, whether in a stain or a paint.

What does the sun actually do to the timber under a clear coat?

Timber is mostly cellulose fibres held together by lignin. Resene's technical paper on clear coatings explains that under UV light, timber loses lignin, voids open up in the surface, cell walls crack, and the surface turns grey and leaves loose cellulose fibres behind. That is the silver-grey fuzz you see on cedar left to weather.

Under a clear coat, the same thing happens, just more slowly and out of sight. The paper notes that even a small percentage of UV getting through a clear coating does the damage, and that the effect is cumulative: 1% getting through for ten days has the same destructive force as 10% in one day. So a coating that blocks most of the UV still loses in the end. The wood directly under the film turns to a weak, powdery layer, and the film is glued to that layer rather than to solid timber.

How does the failure show up?

The same paper describes a classic sequence for clear films on exterior timber:

1. Cracks form in the coating as moisture gets in and the timber swells.
2. UV and water attack the timber quickly at the crack.
3. Mould colonises the interface between film and wood and spreads sideways.
4. Adhesion is lost around the cracks, the film peels back and the timber under it discolours.

On a cedar house that looks like a patchwork: glossy sections that still look new next to grey, peeling, blotchy ones, usually worst on the sunniest and most exposed walls. Satin clears are no better. Resene notes that the flatting pigments and waxes used to reduce gloss have almost zero ability to block UV.

Why can't you just recoat it before it fails?

Because by the time it looks tired, the damage is already under the film. Resene's paper says effective exterior clear coatings over timber need to go on in about 5 to 10 coats, and that when they start to fail, total stripping back to bare timber is required. Adding another coat over a film that is losing its grip only adds weight to a layer that is coming off.

Cedar makes stripping harder than it sounds. It is a comparatively soft timber, softer than pine, and Resene's exterior timber guide notes it is easily damaged by waterblasting and that its only recommendation for cleaning is to scrub or wash. Stripping a failed clear finish off a whole house of cedar by hand is slow, fiddly work, and that is the real maintenance cost of choosing a clear.

What about clear oils?

Clear penetrating oils do not peel, because they do not form a film. The trade-off is that they erode quickly and need frequent reapplication. Resene's paper describes them as eroding so rapidly that constant maintenance is required. That can be a reasonable choice on a small feature panel you can reach from the ground. It is a poor choice for two storeys of weatherboards above a steep bank.

What actually protects cedar outside?

Pigment. Pigment particles absorb or block UV before it reaches the wood, which a clear film cannot do.

Finish	How it handles UV	How it fails	Maintenance reality
Clear film (varnish, clear coat)	Lets some UV through to the timber	Cracks, peels and greys underneath	Full strip to bare timber once it goes
Clear penetrating oil	No film to screen the surface	Erodes and greys, does not peel	Frequent recoating
Penetrating pigmented stain	Pigment screens the surface	Fades and erodes gradually	Clean and recoat, no stripping if kept up
Paint	Opaque film shields the timber	Chalks and eventually cracks	Wash and repaint while the film is still sound
Left uncoated	None	Weathers to silver-grey	Cleaning only; cedar is durable enough to be left to weather

Some modern penetrating stains use extremely fine transparent iron oxide pigments that let visible light through but still absorb UV. Resene's paper notes the slight colour these add is virtually unnoticeable over cedar. That is the closest thing to a natural look with real protection.

One caution: Resene's timber guide says surface-forming stains should be avoided on cedar, because the top layer of cedar degrades quickly under UV and the stain flakes off. The stain needs to soak in, not sit on top. Read the product data sheet and check it is a penetrating stain rated for cedar.

What if your cedar is already peeling?

You have three honest options:

• Strip and go to a penetrating stain. Most work, keeps the timber look. The old clear must come off completely, because a penetrating stain cannot soak in through the remains of a film. Resene's data sheet for one of its penetrating stains says it will not penetrate through existing coatings.
• Strip and paint. Cedar takes paint well once primed with a suitable oil-based wood primer, because its resin bleeds through waterborne primers. Our article on tannin stains on painted cedar explains why the primer choice matters.
• Strip and leave it to weather. Cheapest ongoing, if you are happy with grey.

What we would not do is recoat over a failing clear. It is money spent on a film that is already detaching.

Is a clear finish ever fine outside?

Only where it is fully shielded from UV, such as the underside of a soffit, does Resene consider a clear finish outside. On open weatherboards, it is the wrong product for the job. For more on the options, see our wood staining service page.

If you have cedar with a failing clear finish and want a straight answer on stain, paint or strip, get in touch.