You chose Exterior.
Wood Selection
It's important to first ensure that the species of wood you are using is suitable for exterior use - if you know the species, more information about it's rot resistance can be found here.
Pressure Treated Wood
Some softwoods (often pine) are available as pressure treated or "tanalised" especially for exterior use - this means that they have been impregnated with a preservative treatment at high pressure in order to force the solution in to the boards which helps prevent rot, insect damage and decay. These boards usually have a green or brown colour tint to them. This pressure treatment doesn't usually find it's way to the centre of the boards though, so it's important to treat any exposed wood or cut ends with a good quality preserver (recommendations to follow).
Pressure treated wood in brown (left) and green (right)
The Truth About Exterior Wood Finishes
There is no perfect, ever-lasting finish for wood used in exterior conditions unfortunately as UV damage, moisture and insect damage can and will all take their toll, particularly in woods that are less rot resistant.
But with regular maintenance, it is possible to limit or prevent the effects of such damage depending on use case.
Avoid Ground Contact
Wood that is in contact with the ground is most likely to rot, as that's where the insects live, and where moisture is. Separating the wood from ground contact is always a good idea you could use a paint on product like Bituminous paint.
Black Jack Bituminous Paint 1l https://geni.us/oJHwuf 2.5l https://geni.us/sa9AkLV 5l https://geni.us/PvykE or FixtureDisplays 2.2lbs https://geni.us/5fP7N 11lbs https://geni.us/1tib7 (Amazon Affiliate Links)
Horizontal Surfaces
The next most prone areas to decay are horizontal surfaces, as they are often most exposed to rain water and UV damage.
Wood horizontal surface treated with preservative followed by exterior grade wood paint
The best solution depends on many things like the species of the wood, the environmental conditions, the application, how you want it to perform, but here are a few key principles that should give you the best possible results...
Wood Preserver Treatment
Always use a wood preservative treatment first, paying special attention to end grain (which absorbs moisture much easier than side grain), before applying a top coat of finish. I usually apply 2 coats minimum (and always wait for the previous coat to dry fully before reapplying).
These are clear treatments that absorb in to the wood - once dry, they are completely invisible, but they help to prevent rot, decay and insect damage
Everbuild 1l https://geni.us/CjsBzf 5l https://geni.us/82xvWx Triple Action 1l https://geni.us/vp0P6 5l https://geni.us/J0Ksk (Amazon Affiliate Links)
Sikagard Wood Preserver 5l https://geni.us/B9TRtjU 5 Star Treatment 5l https://geni.us/4xWgU9 (Amazon Affiliate Links)
How To Stop Wood From Turning Grey / Silver - Best Protection From UV Damage
Some people like the way that wood silvers once weathered, others do not.
Silvering effect caused by UV damage on timber
Using an exterior pigment based finish product like a stain or paint designed especially for wood will give the best possible protection from UV damage as the pigments fill the pores of the wood and protect it from sun damage.
Sadolin Stains Natural 2.5l https://geni.us/6eSWZ Light Oak 1l Classic https://geni.us/4GN6f96 1l https://geni.us/69e7GuA 2.5l Classic https://geni.us/isr0J 2.5l https://geni.us/sjGt3 5l https://geni.us/Bc2w7C Natural Cedar 1l https://geni.us/kadAm Ebony 500ml https://geni.us/ISkqWe3 Ebony 1l https://geni.us/Pg7SX9 Black Stain 1l https://geni.us/TEjBBpn Antique Pine 2.5l https://geni.us/XBWB0 Mahogany 1l https://geni.us/FX3eLx Mahogany 1l Classic https://geni.us/yYiUnPr 2.5l https://geni.us/e5Nz9 (Amazon Affiliate Links)
Sikkens Cetol Stains Light Oak 1l https://geni.us/fVzj 2.5l https://geni.us/obUAA Dark Oak 1l https://geni.us/HBEjcG3 2.5l https://geni.us/PuAD6 Ebony 1l https://geni.us/h5eV Walnut 1l https://geni.us/S6uG 2.5l https://geni.us/bUVXRJ (Amazon Affiliate Links)
Dulux Weathershield Paint (various colours) 750ml https://geni.us/hf1RW more colour options https://geni.us/Z2ks6I more colour options https://geni.us/0qT9HGG Black https://geni.us/kYYhH1 (Amazon Affiliate Links)
But what if I don't want to paint or stain the wood?
Probably the next most effective clear wood finish is hard wax oil with UV protection, however please note: it is not suitable for horizontal surfaces where water can sit
Osmo UV Protection Oil Clear 750ml https://geni.us/KZgANkr 2.5l https://geni.us/dfXLm (Amazon Affiliate Links)
Fiddes Exterior Wood Oil 2.5l https://geni.us/Uy9ItFs (Amazon Affiliate Links)
What other options are there?
There are a couple of other types of exterior wood treatments:
1. Oil based products
These are absorbed into the surface of the wood and are very easy to apply and maintain (you can simply reapply as needed) but need regular maintenance. Teak oil is generally the best regarded oil for exterior use.
Bartoline Teak Oil - 1l https://geni.us/wb3H 2l https://geni.us/G1Ziqx 5l https://geni.us/GBglYL5 Teak Oil Bartoline 1l https://geni.us/aY25 2l https://geni.us/sob533C 5l https://geni.us/JuiwWW Liberon - 1l https://geni.us/wn2MkRK Minwax Teak Oil - quart/946ml https://geni.us/0Z6yqJp AquakTeak - 16oz https://geni.us/2r8eqJ Rust-Oleum - Quart / 946ml https://geni.us/ooAj (Amazon Affiliate Links)
2. Those that form a protective coating on the surface of the wood like varnishes
These are generally longer lasting than oil based products, but require more work (sanding and recoating) to maintain. Marine/yacht varnishes are among the more durable options.
Epifanes Marine (oil based) Varnish 250ml https://geni.us/9aAr 500ml / 1l https://geni.us/ZGFHQ (Amazon Affiliate Links)
Ronseal Exterior Yacht Varnish (solvent based) 500ml Satin https://geni.us/FlnWr 1l Satin https://geni.us/Aw6RXyd 500ml Gloss https://geni.us/3aJmLM 1l Gloss https://geni.us/HWgbVD (Amazon Affiliate Links)
Paint Factory (solvent based) Spray Yacht Varnish 3x 250ml https://geni.us/G8Mxv8e 6x 250ml https://geni.us/6LNpHTg (Amazon Affiliate Links)
Rustins Outdoor (water based) Varnish 500ml Satin https://geni.us/pfYusTM 1l Satin https://geni.us/jkyCEm 1l Gloss https://geni.us/kl8V 2.5l Satin https://geni.us/zbc0sOH 2.5l Gloss https://geni.us/tXS4 (Amazon Affiliate Links)
