There is a bewildering array of sealants available at your chandlers for jobs requiring a watertight seal or bond between surfaces.Attaching deck hardware, repairing the inflatable dinghy, making a hull to deck seal, fitting portlights or sealing through-hulls and seacocks all require sealants with special qualities.
Here’s a brief run through of what’s available and what it’s good for:
Silicone.
These are easy-to-use and generally clean products with a variety of uses such as isolating dissimilar metals and for sealing wood, glass and most plastics. They resist most boaty chemicals. Not recommended for underwater tasks such as sealing through hulls or for really tough jobs like hull to deck joints. A bit wimpy on the adhesive front.
Polysulphide (polysulfide).
Fantastically versatile and strong, stay flexible, bond well to most surfaces and can be used above or below the waterline. Not suitable for bonding plastics - melt acrylics and some plastics such as ABS and polycarbonates such as Lexan. Yikes. Take ages to cure.
Polyurethane.Silicone.
These are easy-to-use and generally clean products with a variety of uses such as isolating dissimilar metals and for sealing wood, glass and most plastics. They resist most boaty chemicals. Not recommended for underwater tasks such as sealing through hulls or for really tough jobs like hull to deck joints. A bit wimpy on the adhesive front.
Polysulphide (polysulfide).
Fantastically versatile and strong, stay flexible, bond well to most surfaces and can be used above or below the waterline. Not suitable for bonding plastics - melt acrylics and some plastics such as ABS and polycarbonates such as Lexan. Yikes. Take ages to cure.
The Incredible Hulk of the sealant world! Powerfully adhesive, they cure to form a flexible seal that’s all but impossible to break. There are several brands available with different cure rates, elongation characteristics, and tensile strength. Sika offers a large range of polyurethane hybrids for different specific purposes, Sikaflex 291 being the all-rounder. The universe could be held together with 3M 5200. I sealed a large gash in my Zodiac with this product and it was still going strong years later. Hull to deck joints, sealing through hulls and any other permanent bonding job cry out for polyurethane but don’t use it on acrylics. And don’t use it on anything you might contemplate taking apart again. Ever.
Polyether.
Polyurethane’s better looking, smarter but slightly wimpier brother. Looks good for a long time, cures very quickly, UV resistant, ultra flexible, shrugs off teak oils so can be used as a deck caulk, doesn’t stink and doesn’t shrink. What’s not to like? Oh, and you can use it on plastics, even ABS and polycarbonates. 3M 4000UV is an example.
Then we have a bunch of specialised sealants: Butyls, acrylics and bedding compounds. I’ve never found a use for these, given the availability of the above, but there may be special, obscure applications for which they are more suitable than the mainstream sealant types.
