I may have been the first experimenter to try Butyl tape in the 70s, (on my first of several cruising boat projects)...
I financed this project by working as the service manager (for 3 states) in my dad's Pella window dealership. As such, I had dozens of rolls of the stuff laying around, so used it to bed many ports, deck ports & vents on the boat. Here are the caveats...
It sticks like crazy, FOREVER, and does NOT release. With window glass in a metal frame, (if a window got broken}, we'd use red hot putty knives to cut through the sticky tape. IT IS THE ONLY WAY TO DO IT! Every 5 seconds, I'd put the putty knife back under the torch's flame, to keep the blade red hot.
So, if you have to remove something made of plastic, on fiberglas, (= "fiberglass reinforced plastic"), then you may melt one of these surfaces, in removal. SO GO FAST! Also, it never sets up, but stays the consistency of bubble gum. On a hot summer day, & especially on a horizontal surface, it compresses more with heat, and oozes out of the edge more. If you wipe off or wash the surface, this ooze smears all over the boat, and requires mineral spirits and elbow grease to remove. I regretted having used it!
If you use it, I would use it as a seal & spacer, but, given room, I'd put it well inboard of the hardware edges or flange edges. On the very edge, I'd still run a small bead of caulk around the perimiter, (which does set up), and you won't have this problem. The difficulty of removal, however, remains. On very small pieces it is not an issue, but on large deck plates it is!
Mark AKA, Delphys Marine
Last edited by Mark Johnson; 02-07-2012 at 08:57 AM.