sdowney1 wrote:
I thought I would share this. 5 years ago I finished a mahogany transom
with minwax ipswitch pine and then topcoated with polycyclic. For some
reason it has held up well.

At a guess, your 'ipswitch pine' was a penetrating oil stain, and even
mahogany
can weather a bit better when oiled. Minor porosity in the varnish
would allow some
moisture in, and the oiled wood sheds that small amount of water while
un-oiled
wood might soak some up. The wet/dry/wet/freeze cycling can open up
major
cracks in the wood, and cause it to shed a finish.

Spar varnish (real spar varnish, intended for real spars back before
polymer science...)
was extra oily and gummy (and never really dried out). Folk who tried
it on indoor furniture
found it stuck to clothing... you may have replicated its virtues
without the drawbacks,
by doing those two layers of finish.