First, we suggest doing an inspection around the AC and vents. Any loose caulk needs to be replaced and any areas that are not sufficiently caulked should be. We offer Rubex caulk on our website. This caulk was field tested to be compatible with the Rvroofmagic so it will not void the warranty. We suggest taping off the roof with the blue painter’s tape to allow you to paint a straight line as well as eliminated any drips. Most people will do their cut in work around the ac’s vents etc. first and then go back with the roller over the entire roof