When I worked in boat yards, the joke was that every conversation among the workers and owners eventually came back to adhesives. The same could be said about sealants; the choices are mind-boggling. And what the %^&* is the difference between a "boat adhesive" and a "boat sealant" anyway?
According to Webster's, an adhesive is "a substance used for sticking objects or materials together." A sealant is a "material used for sealing something so as to make it airtight or watertight." It's easy to see why boaters get the two confused, since if we're trying to glue two things together we're almost always trying to make that connection watertight. Common usage distinguishes between the two based on how they are used: adhesives are spread over a large area, while sealants fill in narrow gaps.
Adding to the confusion is that all of us have our pet favorites in each category—and we'll defend them to the death, even as new and better products emerge. So how can you choose the right sealant or adhesive for your specific job? Here are some general guidelines to get you started:
1. What are you trying to glue together?
You are trying to strengthen a physical bond by making a chemical one, and different chemicals do better with wood, fiberglass, aluminum, and stainless steel. Most products will come with a list of preferred materials right in the directions for use; ignore at your peril.
2. How long do you want it to last?
It may seem at first that all repairs should be made to last forever, but if there is even a slim chance you'll need to redo a repair, use something a little less than absolutely permanent. Anyone who has tried to remove cured 3M 5200 without damaging what it's stuck to will agree.
3. How visible will it be?
All adhesives and sealants are messy, but some are messier than others. And some are easier to clean up once they're cured. If your repair is in an eye-catching location, make sure you take enough time to protect/prep the surrounding areas. And consider making the repair with something at the easy end of the cleanup scale.
For more, read Gary Reich's posts:
Resins, Resins, Everywhere, But Which One to Use?
Picking the Correct Marine Sealants
And for specifics about deck caulking, here's a link to an older post that's still valid: