Which sealant should I use?

dlochner · May 4, 2020

Silicone would be a bad choice. Since it is exposed to sunlight 3M 4000UV.

TomY · May 4, 2020

I would use a polysulfide like Boatlife Life Caulk in a mahogany color. Now that you have it sealed, I'd tape both sides of a narrow joint area and apply a bead and tool it off, then carefully pull the tape before it dries. The stuff is messy so use gloves. If you're good with the caulking gun (I'm not), you could skip the taping.

You can then varnish right over this stuff, after it dries (which takes a few days but it gets solid pretty quickly).

Don S/V ILLusion · May 4, 2020

Sikaflex is also good in this application asa polyurethane sealant.

thinwater · May 4, 2020

^^ I'm assuming he means Sika 291. They make a lot of products.

dlochner · May 5, 2020

sailnoproblem said:
Thanks for the information. I will look into Boatlife life calk, and the Sika 291. I have gotten excellent advise from Tom so I am leaning towards his suggestion.
Thanks again for the suggestions, cheers.

Since the seam will be exposed to sunlight, make sure the caulk you use is UV resistant. White LifeCaulk is not UV resistant and will turn an ugly shade of brown over time. The Mahogany colored caulk probably won't be affected as much as it is brown already. Lifecaulk will grow legs and walk around your boat, Tom's comment about it being messy is a bit of an understatement, it is however a fine caulk once it cures. I've used it often.

TomY · May 7, 2020

sailnoproblem said:
I guess what I thought would be a simple question is not so simple. I guess I'll be doing more research. I sure don't want the nasty black/brown color after all the wood prep. I thought Boatlife would make sense.

I'm not sure Boatlife would be the best choice, either. You're application is asking for a small filet like joint of sealer between sealed wood and fiberglass. I recommend the Mahogany Boatlife polysulfide because once cured, it takes varnish nicely. But polysulfide doesn't stick that well to surfaces that aren't prepped for it. That's why I find it a good sealant in the some applications, especially where wood is involved. When you have two mating surfaces, polysulfide has superior gap filling and flowing properties. I heard wooden boat builders refer to this as the 'splooge' factor.

The great thing is, once cured, the material doesn't adhere that well to the sealed wood. it's easy to remove as well as sand off the raw wood. Then varnish covers it all and the joint lasts a long long time (beyond when you'd have to 'wood' the pieces it's sealing.

You have a perfect rubber like gasket between the pieces that seals out water very well. It's not a great adhesive though except to raw wood.

You may be better with a polyurethane type sealant (Sika) due to it's higher adhesion? Either way, taping off for a filet, it should blend in with the toerail and disappear. But it has to be a good seal, and stay there. Tough one!

dlochner · May 7, 2020

I've not used either one and have never used a caulk in your application. Sorry.

Barnacle Bill · May 11, 2020

Boatlife Life Calk also comes in a teak color. They also make a primer to coat the teak with for best adhesion to the teak. It is a good choice, but takes longer to cure than most caulks.

TomY · May 12, 2020

sailnoproblem said:
OK I think I have narrowed it to these two SIKA SIKAFLEX 505UV, or 521UV. I am only looking for a sealant, that is UV resistant. Any thoughts?

I've used a bit of the Sika products. The urethane type caulks stick like glue so I avoid them for most applications. But this adhesion may be just just what you want.

I remember the color match wasn't great but they may have a better option there. Good luck!

dlochner · May 12, 2020

sailnoproblem said:
as I don't want to do this project again

Oh, you will. It's a boat.