Bill, the rubber roof on my motorhome wraps over the side and goes down about 4 to 6 inches to a horizontal seam where it meets with the fiberglass sides. The first thing I did was to tape full newspaper sheets to the seam as the liquid rubber does sag a bit on the sides. I scrubbed the top really well to remove all chalking and loose caulk. I checked to make sure all screws were tight and any holes or tears were repaired and the repairs smoothed over. This probably was overkill, but I was really tired of having a leaking roof. The hardest part was scrubbing around the air conditioners. I did temporarilyremove the refrigerator vent cover.
The catalyst was mixed with the liquid rubber and this was applied to the roof with a roller. A brush was used on the sides and in all those spots the roller wouldn't fit. Once finished, the original rubber roof will swell a bit and you will think that you've ruined it for good.

After a couple of hours of drying, the swelling will start to disappear and after 24 hours, it is all gone.
This stuff is super sticky, and I ended up throwing away everyting that I used in applying it, includeing my shoes, pants, shirt, brushes, rollers, etc.etc.etc.? :
David Derway