For plywood trailer boxes there are many finish options. The one I like to use is Durabak. A good exterior finish needs to provide moisture protection, be durable and endure UV exposure.
In Nov ’07 I made a Durabak test panel and left it out in the weather. The test panel is a piece of 9mm marine grade Okoume plywood. The panel is bare wood lightly sanded with 80 grit covered with two coats of Durabak applied per manufacturers instructions. The back and edges of the panel were left bare. After the 2 days of curing I positioned the piece horizontally outside in full sun.
The panel is almost four years old now; still positioned horizontally in full weather. The piece has endured temperature ranges of low 20’s to high single digits, 100+ degrees, tons of Oregon rain, covered in snow for weeks at a time and through many freeze-thaw cycles then baked in the sun again.
I’m happy to say the panel is holding up fine with only minor fading. There is rotting wood where the panel is not coated, but just fine beneath. I poked around the edges and there is no sign of lifting.
For a trailer stored inside when not in use, I’m assuming one year of full weather exposure is equal to four-six years finish life. Based on my current test, that means your Durabak coated Explorer Box can easily go sixteen years without worry about it’s finish.
Note: I’m not a Durabak dealer and I paid the same price everyone else does. I am just a happy customer of their products.