In that case you want to go for a microporous exterior finish that will allow the wood to expand and contract in different climates - it is a non cracking/flaking product.
The darker the shade of brown you choose, the longer the life of the protection. Either Woodsheld or Butinox 1 will give excellent protection as a result of their trans-oxide pigments which block out UV rays.
Sorry not enough info: The widows are coming with a house kit and are from Nordan which according to their site are double vacuum pressure treated. See here:
However the kit manufacturers site just says that they have a base coat preservative finish, which i'm assuming means as Nordan states on their site. On further investigation on the Nordan site it also suggests microporous paint as a finish amongst a few others.
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