I wouldn't do the whole thing, just the seams. Why add all the weight and risk having it lift off down the road someday or while your going down the road. Every pound counts on a trailer just like race cars.

My roofs have been one peice aluiminum with seams on the edges. I get up on my roofs and wash em at least once a year, that helps keep the black streaks from running down the side of my trailer. Having all that coating up there would be hard to keep clean and who knows what's gonna run down the sides then.