One approach to exterior wood is to abandon the whole idea of
covering it with a waterproof film of paint.  Some houses are clad in
unpainted wood.  What about this approach:

1  infiltrate long-lived antifungal agent (copper naphthenate, e.g.
Metalex®) as a solution in turps which soaks into the wood;

2  oil to repel water.

        One oil I've tried is used engine sump oil.  It's not as poisonous
as I thought it would be, especially since we forced the oil companies to
quit adding lead to the petrol.  Some fungi quite seem to like the new less
toxic sump oil!

        Anyone else tried this non-paint approach?

R