Clear exterior oil/varnish that lasts?

I've tried quite a few different clear treatments that are meant to be for outdoors and none of them seem to last a full year outside.

Does anyone know of a good clear treatment that will last a couple of years against the elements without regular maintenance?

ive tried lots of different things on outdoor tables and benches that are made from green or part dried timber ,, i think the problem is the moisture coming out of the wood pushes coating off the surface,,

a customer of mine uses rubio monocoat for indoor items,it will only take one coat as it seals the surface and goes on like oil , they also sell a product for outdoor use ,, its not cheap,,

We're doing some work with Cuprinol at the moment, so I now have a decent understanding of why things don't work, not that this helps much. The main problem is flexibility as the wood shrinks. Have you tried Endeavour Oil or Le Tonkinois varnish? I've tried the former and liked it, not tried the latter yet.

Alec

It can be when the wood is not dry but I have used many clear treatments on dry wood and it never lasts a full year without flaking.

There is a huge range of indoor clear treatments and these work well already.

But it's an outdoor one I'm after. Sadolin now do this Sadolin Extra Durable Clear Coat - Clear Satin (1L) 241280

which I'll give a go next.

We're doing some work with Cuprinol at the moment, so I now have a decent understanding of why things don't work, not that this helps much. The main problem is flexibility as the wood shrinks. Have you tried Endeavour Oil or Le Tonkinois varnish? I've tried the former and liked it, not tried the latter yet.

 

Alec

Tried Le Tonkinois and it's not bad although still darkens the wood a little. It's my preferred choice of treatment at the moment though.

Without going into too much highly technical detail, the challenge is obtaining a combination of adhesive and cohesive strength, together with abrasion resistance. Adhesive strength is the bond between the surface and the coating - poor adhesive strength results in flaking. Cohesive strength is the internal bonding in the coating layer - poor cohesive strength results in crazing or powdering.

The surface of the wood is constantly expanding and contracting, dependent on the external humidity. Water uptake swells the cells so the wood expands. This creates stress between the wood and the coating which doesn't swell. It then fails either adhesively or cohesively, depending on which is the weaker bond.

The alternative is something which impregnates the surface cells, such as an oil, or doesn't permanently bond so can move with the wood, such as a wax. The down-side is that these are soft, so you don't get the surface finish to start with and then they oxidize and erode fairly fast. The best performing compromise down this route is the tung-oil based materials such as Endeavour or Le Tonkinois. These are used in marine applications, e.g. wooden ships, and are UV stable, hence the rate of erosion is relatively low, but they are expensive.

Current thinking, as per Sadolin Extra Durable, is to go for cohesive strength and form a high strength encapsulating barrier layer. In theory, if no moisture can get in or out the wood doesn't change dimensionally and hence the coating is not stressed and doesn't fail. If you get a perfect film, these will last a reasonable time, but all organic materials have a reasonably high permeability to moisture, so it will fail in time by flaking.

This is one of the areas we're currently trying to address, by developing an impermeable (inorganic) solution.

Alec

The reason alot of clear coatings dont work outside is because in alot of them there is nothing to filter out uv rays, the best exterior coatings have a small amount of colour to them, so try something like sadolin classic as a base stain, this is a low build wood stain that penetrates the timber, then as a top coat use sadolin extra this is a medium build stain that surface sits and protects the surface. really hard wearing and used in this combination will last a good few years. from memory liberon do some really good exterior danish oils that have uv protection in them, other than that yacht varnish is a really hard wairing finish. hope this helps

 

This is one of the areas we're currently trying to address, by developing an impermeable (inorganic) solution.

 

Alec

Thanks for the indepth answer Alec. The Sadolin sounds good but also like the wood has got to be properly dry.

While I think I'm right in that Le Tonkinois is micro porus and so you can get away with some moisture in the wood.

2nd vote for Tung oil. I have been using Liberon Wooden worktop Tung oil on my sculptures, works really well but it is expensive - used on our beech work top round the sink it has lasted 4 years so far and looks great.

Lots and lots of coats of Linseed oil, first few coats diluted 50/50 with Turpentine works well on greenish wood too and is cheap but a ball ache to apply.

The reason alot of clear coatings dont work outside is because in alot of them there is nothing to filter out uv rays, the best exterior coatings have a small amount of colour to them, so try something like sadolin classic as a base stain, this is a low build wood stain that penetrates the timber, then as a top coat use sadolin extra this is a medium build stain that surface sits and protects the surface. really hard wearing and used in this combination will last a good few years. from memory liberon do some really good exterior danish oils that have uv protection in them, other than that yacht varnish is a really hard wairing finish. hope this helps

Thanks Mark. I know there's lots of long lasting stains out there but it's a clear finish I'm after.

I've used all the Danish oils - Rustins is the original and I've found the best but as Alec says it breaks down outside. It doesn't flake but more leaves streaks....

Incidentally Rustins do a garden furniture oil with added UV filters but again this still breaks down over a year. Then they do an outdoor danish oil. This is a totally different product to normal Danish oil and is water based but again needs replenishing 3 times a year.

Normal yacht varnish is good but can leave a thick coat and again you can't put on wood with any moisture left in it.

Still looking like Le Tonkinois.... !