re: Outdoor Finish

As to the high end spar varnishes and such, do a search on short and long oil finishes.

Most the high end (e.g., hundred bucks a gallon) finishes are long oil finishes.

Calling something a long oil finish is just a way of saying the finish has more linseed or tung oil added to the resin and solvent mix. Obviously, then, a short oil finish has less.

Adding oil to the finish produces a finish less resistant to abrasion, but more flexible. As such, it has more capacity to flex with changes in the wood caused by moisture gain and loss.

Of course, we need to be mindful of that everything has its limits and everything needs maintenance.