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RossC32 09-29-2011 02:12 PM

Butyl Tape and leaking port-light
After reading Maine Sail’s contribution on butyl tape, I thought it might be the way to solve a leak I have in an existing port light assembly but I want other people’s opinions on this since I have never done a job like this.

The assembly comes in two parts. The window unit is an aluminum frame with the glass well sealed into it. No leak there. The frame has a ~1/2” “flange”, can’t think of a better word, all the way around the inside at right angles to the frame. This flange fits into the window cut-out of the cabin, or in other words, in the hole that was cut out of the fibreglass. This window unit is held in place by frame inside the cabin, which is screwed directly on to the outside frame.

The leak comes from where the outside frame butts against the fibre-glass of the cabin. Whatever caulking had been used has failed and I was thinking of re-caulking it with the butyl tape.

Problem one is that there are small but obvious gaps between the outside aluminum frame and the cabin surface.
Problem two is that the inside of the cabin is a fibre-glass liner and so the window assembly is sort of clamping the liner to the deck/cabin structure. There is probably some movement between them.
Problem three is that the previous caulking was a sort of rubbery, translucent kind of stuff that stuck mainly to the aluminum and not the fibre-glass of the boat. Does this sound like some kind of silicon-based product?

From my too-long description, does it sound like butyl tape is the way to go? If not, any suggestions?


Maine Sail 09-29-2011 02:31 PM


I've done exactly what you describe and it works very well. However you may need to double up the thickness of the tape to do so. You'll also want to be 110% sure the cabin sides are clean and free of silicone. This may involve wet sanding.

Some tips.

1- Preset the port into the cabin and then tape very precisly around EXACTLY where you want it to be. I find the blue or green painters tape to work well..

2- Remove port, make sure both surfaces are 110% clean, and do a lap or two with butyl and then CAREFULLY align it with the taped perimeter (hoovering though) and then in one motion press it firmly into place. Once the butyl hits the clean gelcoat there is not much room for "adjustment" so lineit up well the first time.

3- With someone holding the port in place go into the boat and begin installing the trim ring and screws. Coat the screws in Tef-Gel before screwing them into the aluminum port.

4- Tighten the screws a little bit at a time. DO NOT be tempted to squish it all out at once, you can't without damaging things or stripping or snapping screws.. You can use pressure from the outside to compress it to then go back in and take a turn or two on the screws. It can take a day or two (in warm weather) for the butyl to fully compress and be squeezed into all the nooks and crannies. In colder temps heating the port frame will help..

5- Remove residue by making a super ball sized piece of butyl and working at it with a fast stab n' pull motion. Clean up what you can't get with that technique with a damp rag with mineral spirits and you;re done.

6- Give it a few days and if some is still squeezing out re-tighten and clean up ooze.

RGBrogan 09-29-2011 05:24 PM

Cabin liner fill
I followed Main Sails instructions and I just completed a project where I replaced all 6 ports on my H34. (thanks for the great help, I never thought I could do that until I read a bunch of Main Sails articles)

Main Sail -- this is your page right?
Installing New Found Metals Stainless Portlights Photo Gallery by Compass Marine at

Anyway, at first I tried to seal 2 of my new ports with polysulfide calk and I realized that I did not have enough skill to get a good caulk bead. So I switched over to using some gray butyl tape I purchased from Main Sail. It worked very well as did the ball/ stab n' pull method to remove all the excess material. I used a dull putty knife as a blade to score a line in the butyl before I pulled the excess away. It looks very professional now.

Insofar as your cabin liner goes, I had a similar problem. To strengthen the area between the hull and liner I used expandable spray foam from Evercoat. The foam is also water resistant so it will further to help seal that area of your cabin.

To apply it I used masking tape to seal off the inside of the port and sprayed it from the outside. Make sure you mask off the port because as the foam expands it may want to drip into the cabin and clean up is not fun. Also, make sure you support the liner and the cabin side with some sort of clamp as the foam may want to expand outward and distort your liner.

After the foam is dry I used a Dremel with a sanding bit to smooth out the opining. (once again it is a good idea to keep the inside masked off as this step produces copious amounts of dust).

With the opining all smooth and ready to go I put the new ports in.

The one trick I learned with the butyl tape is that I cut it to size and applied it one section at a time vs. trying to wrap one continuous loop around the trim ring. I.E. I would cut a pieces for the top, left, right and bottom sides, bed them down with the wax paper still attached and once satisfied I removed the wax paper and worked the butyl together to fill any gaps.

After I did that project I used some of the gray butyl tape to re-bed most of my deck hardware and a deck hatch. So far I have not had any leaks and I am sold on the material as a great beding material.

Good luck with your project!

ilikerust 09-29-2011 05:56 PM

I have the same port lights, if I'm understanding your description correctly, on my 1968 Pearson. I re-bedded two of them that were leaking, using butyl tape, after reading all of Maine Sail's excellent tutorials. The previous owner had bedded them in BoatLife stinky caulk. What a stink that made, scraping all that crud off!

Anyhow, it worked extremely well. Nary a drop since.

ilikerust 09-29-2011 05:59 PM

By the way, make sure you remove every last bit of that old sealant - it does sound like silicone, based on your description. I used a utility knife blade to scrape away all the old adhesive from the aluminum frame and the perimeter of the fiberglass opening, followed by sandpaper to really get it clean, followed by a solvent wipe.

MastUndSchotbruch 09-29-2011 06:08 PM

not so much luck with chainplates
Hm, to add a negative note, I rebedded all my chainplates with grey butyl tape but most of them are still leaking. The holes in the deck where pretty big (chainplates not well fitting) and it was hard to exert enough pressure with the cover plates to squeeze the tape into shape (the plates are held by small screws). I am considering taking them all out again and redo with polysulfite. Big disappointment!

Maine Sail 09-29-2011 07:05 PM


Originally Posted by MastUndSchotbruch (Post 781065)
Hm, to add a negative note, I rebedded all my chainplates with grey butyl tape but most of them are still leaking. The holes in the deck where pretty big (chainplates not well fitting) and it was hard to exert enough pressure with the cover plates to squeeze the tape into shape (the plates are held by small screws). I am considering taking them all out again and redo with polysulfite. Big disappointment!

Butyl works best in compression and it sounds like on your boat you can't get enough compression with those big holes, and only screws not through bolts. Probably not the best fit for butyl.

I've bedded lots of chain plates with butyl, and none have leaked, but I choose them carefully. I don't however use butyl on all chain plates and base my decision for its use on the particular design I am working with.

The original chain plates on our CS lasted 31 years without leaking. I only pulled them to inspect them. I then rebedded them with butyl and they've been bomb proof. Our chain plates are a different design though and the plates are through bolted...

Butyl is not always ideal for every bedding situation so don't let one situation get you down on it.

MastUndSchotbruch 09-29-2011 11:51 PM

Yes, I am glad you agree. In my situation it is hard to apply pressure which makes butyl an inefficient choice. Live and learn!

baboon 09-30-2011 12:31 AM

I recently re-bedded my through bolted ports with butyl, worked well. We have had a few big fall rainstorms with no leaks yet. Most of the points have been made, but...

I agree with "cutting" the butyl ooze with a putty knife prior to the stab and pull step. Otherwise there is a tendancy to pull some butyl out of the seem.

Get all of the old stuff off, it will be hard if it is silicone. I used Mainsails method of a sharp chisel used very carfully at a low angle to get it off the gelcoat followed by wet sanding and a final solvent wipe.

Use lots of butyl, it is cheap. It will slowly ooze into the nook and gaps if you do it on a hot day and go slow. It is much easyer to clean up than most caulk.

The stuff sold through his compass marine site really is better than what was available at my local RV dealer. It is the proper thickness for port work and although stickyer, is easyer to handle because of its size and packaging. I found that the corners of the ports tended to get starved of butyl, and a double layer helped, the strait sections did not need as much, your milage may vary.

RossC32 09-30-2011 11:55 AM

Thank you all, very, very much for your contributions, responses and suggestions. I am now actually looking forward to the project instead of dreading it.
Cheers and thanks again,

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