Hey Pete,

I've built boxes with both methods - sealed/primed and epoxy. Epoxy lasted MUCH longer. However, to save cash, I'll be building my next one with polyester resin.

Also, to save weight, use "door skin" instead of plywood. Cheaper and easier to work with. Once you put on the resin, it gets pretty stiff. I braced the inside with 1" x 1" pine (also coated with resin), and sealed corners with resin/microballoons. Sanded and painted the outside.

Could still jump on the top after 1 1/2 years of being stored outside. In the summer rains, the water did pool inside occasionally (loose opening seal in the back due to my underwhelming carpentry skills), and still didn't really rot (except where I drilled drain holes and didn't seal them)

Whole project cost about $100