If it has 3" on it now I would go with the 4". I used the 4" to seal the roof seams where it meets the front and back caps all the way across. Be aware, this stuff is like gorilla tape once it sticks you are not going to get it back off so take your time.
17 Chevy 3500 DRW 4X4
16 Mobile Suites 36RSSB4
2 Mini Dachshunds
I used the 4 inch to reseal my slideouts. It worked great but I would recommend finding the right roller used to lay the tape down. I did not have one and felt I could have gotten a better bond with it.
George and Debi
2006 F250 PSD, 2005 PF30CK
Is it better to take off the old tape or just go over it. I have a few bubbles under the old tape. It is not leaking any where. I just want to retape as the 5er is going on 7 years old. Just trying to stay ahead of the game. Thanks