I brushed mine on, but I only did the seams and protrusions on my 30-foot Allegro. It has an old-school aluminum roof so I didn't need to do the whole thing. I used less than a gallon.
The warmer it is outside while you're applying the quicker it thickens. It is thick enough not to run if you're careful. I applied mine with a cheap paintbrush. I didn't have to tape anything off - it's thick enough to stay where you brush it on.
I didn't use their primer. I wire-brushed the loose stuff off and washed it with soap and water (Dawn to be specific).
It's been a year now and it's still stuck like glue, really flexible and doing is job. Really good stuff.