Butyl tape has been used successfully for years, and has numerous advantages. With adhesive strength similar to duct tape and about a tenth of the tensile strength of even the weakest curing sealant, butyl tape doesn't harden, but remains sticky and pliant. While that makes future disassembly a snap, butyl tape is still less likely to break its seal because it stretches. If a gap does open up, the still-sticky tape self-heal.
Cutting, fitting, and molding butyl tape can be more labor intensive than squeezing on a squiggle of Life-Caulk, but tape is far less messy, eliminating the need to mask. Compatibility with plastic, combined with a gasket-like and uniform application, make butyl tape the most reliable sealant for framed portlights. Butyl tape doesn't cure, so you can take all the time you need fitting hardware. Success is less dependent on technique than with the curing sealants. It's easier to control, seems unaffected by UV exposure, is relatively low in cost, and has a shelf life measured in years rather than weeks.
All is not completely rosy, however. The malleability of butyl tape changes with temperature, making it difficult to use in cold weather and prone to compression creep in tropical heat. Permanently sticky edges capture dirt. Butyl tape is not an adhesive — it can be used only for sealing hardware bolted in place. Don't use it with screws because rotating them breaks the seal. Petroleum products dissolve butyl tape, so it is a poor choice around fuel fills or vents. Likewise, its modest adhesive strength rules it out for use below the waterline.
Even for the uses where butyl tape clearly excels, the biggest problem is picking the right butyl tape. Primarily a construction product intended for sealing siding, windows, and roof panels, it's manufactured in hundreds of varieties, comes in a coil, not a cartridge, and is unlikely to be found at a marine supply store. The usual retail source for the individual user is an RV store. Even here, expect to find more than one type. Black is a no-no because it stains, but how do you decide between the soft one and the harder one, between the thin one and the thicker one, between the gummy one and the one that feels less adhesive? If you have experience with butyl tape or you're confident that you'll know the right product when you feel it, fine.
The application of butyl tape differs from the curing sealants. Here it will be better to apply the tape to the base of the hardware. Start by tearing off small pieces of butyl tape, rolling them into thin strands, and wrapping them around the bolts just below their heads to fill any gap between fastener and hardware. Insert each bolt into the hardware (but not the deck), and wrap a much thicker rolled strand of butyl tape around the bolt where it exits the hardware base. Use your fingers to compress the tape into the threads and against the underside of the base. Create a fat butyl rubber cone that will more than fill the chamfered hole in the deck.
excerpted from a BoatUs article