I got a gallon of white elastomeric roof coat for mine. Easy, cheap & you can see what's going on underneath easier than with the black asphalt emulsion later on. Here's the deck prior to coating with the sealant. I did 2 coats of urethane, then 2 coats of the roof coat prior to flipping it over.

I figured it was easier to coat the panel first, then flip it over for the build to start. Once the sides are screwed down, the holes along the bottom edge just need to be touched up & sealed with a paint brush. Since I'm doing the build on the frame, this was the simple method (KISS). I screwed in the sides from the bottom of the deck & up into the sides with pilot holes, 2 1/2" drywall screws & Titebond II.
I pre-finished the top the same way, as well as the side panels. Easier to coat things when they're laying flat, no runs to deal with this way.

A slot is milled into the edge to set the side panel in place. Keeps it where it needs to be when the glue is setting up & the screws are run in to hold it all together. Simple & very solid joint with the glue when it sets up.
