I have used a good heavy spar varnish on outside wood with good results. It does need yearly maintenance though.
My MO is to start with good sealer and then 5-6 coats of spar (buy from a wooden yacht restorer).
Every year lightly sand the coat and reapply a new coat or 2 always keeping what looks to be 5 coats on top of the wood.
Speedy
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1983 944 guards red with 16" Fuchs, Host of Wisconsin area timing/ balance shaft belt tensioning party
1987 944S Purchased from Legion. Corvette LT-1 V-8 conversion with Mega Squirt II
Check on progress --->
www.porschehybrids.com/gallery/speedracing944
Favorite Road =
www.tailofthedragon.com 318 turns in 11 miles (11 min 20 sec best run)