LurfysMa wrote in
:

What about a tougher polyurethane? Is there a marine version that is
more weather resistant?

Spar varnish would be your best bet. Normal varnishes (poly, etc)
are quite hard & brittle, and the temperature cycling that occurs
between daytime sun and nightime darkness causes them to crack,
which then lets moisture get behind and they rapidly degrade.
Spar varnishes are softer and can flex with the temperature
cycles.

Note that a good spar varnish, like Interlux or Epifanes, is by
no means cheap.

But nothing lasts forever in an outdoor setting. You'll have
to revarnish every so often whatever you use.

John