Z-Poxy finishing resin is sooooooo easy to sand. Plus epoxies in general have vastly longer shelf-life than polyester resins, there's no reason not to go with epoxy.
Someplace I worked, there was a younger guy working there that had previously worked for a company that used polyester resin. The story was that they tasked him with disposing of a 55 gallon barrel of outdated polyester resin. I his youth and enthusiasm (I suppose), he mixed in the appropriate quantity of MEKP and stirred it well. After the flames and smoke subsided, he was told to seek employment elsewhere. And so, he came to work for the dipchits where I was employed.
Steve