Mahogany Runabout Caulking

Okay, not a sailboat. But y’all are my people and I need some advice. 

I’m about to prepare the seams on this for caulking. I’m planning on using Teak Decking Systems SIS440 in white. 

I’ve heard that you should lay down tape in the seams under the caulk. Why?

This seems to be an enormous pain in the nether bits and I cannot imagine why you’d need to do this. 

For reference, this is prepped for stain (oil based gel stain) and I plan on staining, 2 coats varnish, caulking, sanding, then many more coats of varnish. But I started trying to tape the seams (both real and faux) and it is the most fiddley task ever! Can I just skip that?

I think that the idea is to keep the caulking goop from bonding to the bottom of the seam, which reduces the force needed to stretch it horizontally, thus improving the odds of its sticking to the sides of the seam.  


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Working to overcome my inner peace
The tape allows the caulk to move more in expansion and  contraction of large decks so it doesn't fail on the edges.

I don't think you have much movement on your deck, and certainly not any of the faux seams.

But I'd recommend running all the seams with a seam sander made from 3/16"  alum and sandpaper both sides to fair out the seams.

Gives a nice finished look.

If you do tape use one of these and 1/4" fine line plastic tape.

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Yes it to release the bottom of the seam so you are only glued on the sides.  If not and there is significant thermal expansion it will likely pull away from one side. TDS has amazing elasticity and sticks very very well but there is a limit. If you think about the wood pulling on two sides of a square, how much give there will be, now do the same thing but with the bottom static.  The fine line tape works fine as a release agent.  You will also really be happy if you ever have to redo a seam as cutting out is very easy with the bottom taped.



Super Anarchist
I would definitely agree a) with Sass and b) tape will be fiddly. Which is why GENERALLY we still use traditional cotton/oakum hammered in and then the caulking- sort of a combination between old and new school. Once you get into it you will find using a making iron will likely be faster than trying to get tape down in the seam- and be more 'traditional'...

EDIT: I should note that it is a little hard to see the depth of your seams in that photo... if time or even just initial build thickness won't leave room for a traditional caulking, my above recommendation will have to be re-thought.

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It is how traditionally laid decks are done  so that the caulk does not adhere to the bottom, only the sides of the seam and does a better job of handling expansion and contraction.  I knew a really good old hippy shipwright in the West Indies who used cassette tape - he spent alot of time pondering the music that was on the tape and whether or not it suited the boat, the owner and the overall vibe...

In other construction trades, where sealing a joint that might move a bit, there’s something called ‘backer rod’, which is often a closed cell poly foam rod.  It’s stuffed into a seam and is slightly compressed.  THe goop sits on top of it.  It’s round shape gives the goop a necked shape which makes the mass of goop more compliant and more likely to stay stuck.  Probably not useful until the planking is 1/2” thick, which would introduce other issues.


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