I think you shouldn't consider varnish at all. A good coating of
varnish requires the wood be properly constituted, and if it is oily
(your replacement mahogany door may very well have been oily) or wet,
you will not get a lasting surface. Further, every varnish I have seen
in many years requires multiple coats (at least two, often as many as
eight), and it sounds like most of the posters here are trying to get by
with a single coat. Incidentally, spar varnish is designed for use on
the spars of a sailboat (such as the mast), where there is not a lot of
traffic. It is softer, and will not last in places where there is a lot
of traffic. I used spar varnish on my exterior doors with very good
results, but that was with multiple coats. I used it because I had some
already that I had used on a boat. UV is no longer a problem, as modern
varnishes include UV protection, but one should check the label to be sure.

I think you would do better with a stain that includes UV protection.
They are easy to apply and I have never seen one peel, however, they do
require re coating periodically.

Any clear exterior finish should be viewed as temporary, and renewed
periodically. Paint is longer lasting, but even the best paint will
eventually require replacement.

Don Wiss wrote:
I have a door from my kitchen to the deck. It had been an ordinary wood
with a clear finish. It opened outwards. When it rained I would leave the
door open. For years the finish was fine. Then I upgraded to a mahogany
door. Whatever finish the fellow put on blistered on the outside in a year.
And I haven't left this door open in the rain at all.

So what is the most lasting finish? Thinking like a boater I would think
that spar varnish would be good. But that was years ago. So what should I
use?

Don www.donwiss.com (e-mail link at home page bottom).