I had been spraying my boxes with the water based flat black paint with 2 coats and then spray 2 coats of duratex over that. That's it. No sanding between coats needed due to the texture of the duratex. I did roll the duratex on once and it turned out well but spraying is much faster if you're doing that many enclosures. Btw I use a cheapo harbor freight texture spray gun that was less than $20. A gallon of duratex will easily cover the boxes that you listed.