Learnt my lesson with Linseed oil the hard way . Western red cedar boards on a house as siding and coated with boiled linseed oil , a laborious job. When the sun shone on it it turned black with a fungus that I could not remove bar sandblasting . Gave up and sold the place.
Tung oil might be better as it has driers added , linseed does not.
Koppens make a range of products for such jobs .
Epoxy based products will chalk up in no time exposed to the sun.
The clear coats with UV screens in them would be my choice. Intergrain makes a good one ,water based too so I would make a copper sulphate solution and paint it on first ,let it soak in then use an Intergrain clear coat or 2 which will dry over .
Another option is a strong solution of concentrated radiator additive (the green one) and boric acid(borax) mixed and heated, then painted on when cool. I´ll see if I can find my old formulas . We can´t buy rot preserving chemicals here so must make up with what is available and test to prove. Then a protective clear coat
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