Minwax poly WILL work on mahogony, but you don't want to use it for
outdoor projects. I like to use a marine varnish on outdoor
projects, because 1)it has UV protectants, 2)it's flexible, so when
the wood contracts in the winter and expands in the summer, it will
flex with it, and 3) it's waterproof.
On top of all that, the marine (captain's varnish, i think) varnish I
use is oil-based, so when it penetrates, it accentuates the grain
without raising it, like water-based products will do. This means
less sanding for you!! It's a win-win.