In my opinion, if you are going with MDO, you really don't need to epoxy the plywood surfaces. You really only need to seal the edges and anywhere that you put in fasteners or make holes. The stuff is designed for outdoor signs, it holds up very well. I used it on my boat for the floor, I just painted the edges , caulked the seams and then painted the floor with oil based floor paint with anti skid sand mixed in. Done
All epoxies are 2 part, US Composites is very good to work with, if you aren't sure what you need they are very helpful on the phone to steer you toward the proper product for your application.
Have fun!
-Rich