Marine sealants and adhesives are a bewildering lot. Maybe it’s just me? 4200. 5200. 4000. Dolfinite. TDS. 3000. LifeCaulk. Sikaflex. Silicone. How the heck are you supposed to choose? OMG.

A few things to think about when you’re confronted with this overwhelming array of options. (And these are just the ones that come in a tube. There’s epoxy and butyl, wood glue and even hot glue as other possible options. Oy.)

Where are you using this? Above or below the waterline matters. If it’s below the waterline, the tube ought to say “for use below the waterline.” I’m not usually a believer in “It Must Say Marine” to be used on board, but in this instance I’d definitely shy away from general stuff you buy at a big box store. Simply put, anything below the waterline is something you are trusting your life with.

Does this have a specific application? We just replaced the glass in our portlights with acrylic, and the instructions on installing that specifically say to use silicone. When we rebuilt the forward hatch, with teak strips on top, we used the same kind of goop you’d use on teak decks. Read any instructions carefully.

Are you after sealant or adhesive? Generally, the tube will say one or the other - or they’ll be hyphenated. Which comes first is a bit like the ingredient list on a package at the grocery store. A sealant/adhesive will seal better than an adhesive/sealant. 3M products have a helpful scale that gives you a level between “permanent” (adhesion) and “flexible” (sealant).

What are you bonding to what? You might be bonding wood to wood or fiberglass to fiberglass. Or mixing your media - wood to glass, or metal to wood. Or metal to glass. Make sure whatever you’re using is compatible with BOTH surfaces!

Which side will give up the ghost first? If you’re installing something metal onto something fiberglass, the glass will give up before the metal will. Choosing an adhesive that is too aggressive may mean that when it comes time to take it all apart again for rebedding (and yes, it will have to happen at some point) it’ll destroy the fiberglass in the attempt. If you choose a compound that’s got less adhesion, you may develop leaks before you want to. Make sure you understand this tradeoff.

Basically, marine tubes of goo come in different strengths of how well they adhere. There’s different chemistry involved too.

So what did we use where, and why?

Lifecaulk (polysulfide): Boom gallows hardware (wood to metal). No leak potential in terms of getting us wet, but we’re trying to keep any moisture out of the wood.

Silicone: ONLY to fix the new “glass” in the portlights. Silicone anywhere else is a cleanup nightmare as it will repel any coatings you may want to apply (like paint).

Sikaflex 291/291 LOT: Polyurethane. Relatively tenacious adhesion but not permanent. Fillets on the bulwarks (no sealant, just adhesion), installing the taffrail (wood to wood)

3M 3000 UV: Polyether. Relatively low adhesion, so used for things we think we’ll have to take apart with some frequency. Used this for large surface areas: chainplates and hawseholes (bronze to wood or bronze to fiberglass), though the fasteners were done with more tenacious stuff.

3M 4200: Polyurethane. Tenacious but not permanent. As in, we should be able to remove whatever was installed without destroying the underlying surface. Fasteners. Stem fitting (bronze to fiberglass, below the waterline, painful to access for removal)

3M 5200: Polyurethane. PERMANENT BY GOD. Adhesive as hell. Understand that you will very likely have some reconstruction required if you need to remove something put on with 5200. Cover board (wood to fiberglass)

Butyl tape: Not a marine goo, doesn’t come in a tube. Flexible and sticky forever. We used this to affix portlights into the cabinsides and install the compass and instrument panel.

There’s no one product that will do everything, unfortunately. Understanding what you’re putting together and what you’re looking for will go a long way to helping you decide what will work for you.