definitely prefer ducks back as its a skin thats waxy and fully waterproof that stops the wood moving [once settled no real expansion or contraction]
my shed is 15 year old left the ex 5" pressure treated shiplap about 3 years un finnished to settle down
2 or three coats every 3-5 years after 15 years looks brand new happy to leave now until it starts to looks worn then probably 2 coats at 20 years
any treatment that seals the surface will stop movement where as movement can happen iff a treatment allows water to enter before stopping it penetrating so allowing excessive expansion that is not good in general for building but ok for fences
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