I used to powder coat for Salsco a large Land Scaping Equipment manufacturer. We had a full massive setup with over head cranes, train tracks etc. Huge oven the size of most peoples shops that automatically fed parts in on one side and comes out the other. We degreased and coated everything from tiny parts to full size tractor trailer frames. The cool thing was we did a lot of sub work that really needed a good commercial coater, your right they are not that common. I bet if you can get the word out you can get clients easily.
Personally tho the worked sucked balls, heat was something way beyond screen printing with the oven size we had, worked nights and early AM mostly to survive. Degreasing the metal was critical, nothing sucked more than having a large frame come out of the oven with enamel dripping off. From what i remember corners on bent sheet metal tended to be difficult to achieve, a lot of times the enamel would shrink away from the corners leaving the metal exposed.
