I'm guessing you are asking about the seams where a rubber roof membrane meet a vent or air conditioner or the like. Many people recommend laying a strip of Eternabond over the seam, after cleaning the area with acetone. Once Eternabond is stuck, it is stuck. It will stick well to old sealants that have been cleaned; it should come with application instructions. Here's a link to Eternabond http://tweetys.com/eternabond-roofse...ape-4-x50.aspx
; it can be bought many places.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch