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Fighting topside leaks

Mar 6, 2013

Dick de Grasse photos

Those of us northern sailors who have been out cruising in southern waters for a while know that leaks around ports, chain-plates and other deck fittings can be real pains in the neck (plumbing leaks are a different story). In fact, topside leaks cause us more grief than leaks below the waterline. Of course we only discover topside leaks when it rains or when we hose down the boat. When we leave the boat on the hard for the summer the leaks persist, without us knowing about them, until we discover an unusual amount of fresh water in the bilge and streaks on the interior teak when we return. We have tried everything: 3M Sealer, Boat Caulk, GE Sealer and all (we never use 3M 5200 or 3M 4200 for fear of cementing a leaky port, chain-plate or cleat in-place never to be removed or resealed!). Traditional marine sealers leak much too often especially when exposed to the weather and ultraviolet (UV) rays for long periods of time.


One of our sailing friends asked, “Why can’t we use the same stuff they use to seal automobile windshields?” Automobiles use butyl rubber tape sealer or butyl rubber gaskets. The stuff lasts forever (visit a automobile junkyard and try to find a leaky windshield).

A Google search eventually turned up butyl rubber in several forms. What I was looking for was one-inch-wide, double-stick tape, perhaps 1/8 thick. You can buy butyl rubber in a tube from an auto store, but it comes only in black and is nearly as messy as regular marine caulking. I wanted tape that would stay in place as we put the pieces together and not run all over the place. I discovered a company in Arizona named Best Materials ( They sell all kinds of butyl rubber products mostly used as gaskets and sealant for roofs. A gray, 50-foot roll of one-inch-wide, 3/32-inch-thick, double-stick tape costs $5.75. The shipping costs more than two rolls! We decided to try it.

We replaced one of our four cabin ports with new smoke-tint quarter-inch Lexan. (We took the old, badly crazed, acrylic port to a custom glass place and told them to copy it in Lexan). After reasonably good luck, and no mess, installing the new port using the butyl tape, we sprayed it with a garden hose; the result: no leaks! We immediately decided to do the other three ports.

We cut the tape lengthwise to be a half-inch wide, and used it to seal the inside and outside aluminum trim and filled the barrel-bolt holes with conventional GE Clear Sealer just in case. So far so good. Best of all the butyl tape is easy to apply, comes in a roll, stores ready to use (won’t dry-up in the tube like all the other products we’ve tried), no leaky caulking that sticks to everything, including the crew, and no mess to clean off the ports and interior teak. It seems to last and never lose its flexibility.

After we tightened the trim barrel-bolts, we trimmed off any overflow of butyl tape with a plastic credit card so as not to scratch the Lexan. We’re so confident we even did our leaky chain-plates and saved lots of tape for other deck hardware.

Once in a while we find something new that works. We now carry a big roll of the stuff. We plan to use it for all our weather deck fittings and anything else we can think of. We’re even thinking of butyl rubber gaskets around the head in place of the leaky “O” ring on top of the Y valve, for example.

Dick de Grasse and his wife Kathy live aboard their Tartan 34, Endeavour, in Florida during the winter and in Islesboro, Maine, during the summer.

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Mar 6, 2013 04:16 pm
 Posted by  oenghus

3M makes a butyl rubber product electricians use. It comes in two inch squares. I am not sure it is available in a roll. I have used it several times to caulk leaky shrouds on different boats. No more leaks.

Mar 6, 2013 04:17 pm
 Posted by  glen


(we never use 3M 5200 or 3M 4200 for fear of cementing a leaky port, chain-plate or cleat in-place never to be removed or resealed!)

You suggest that using 3M5200 will cause the stainless part to be stuck forever to the sealant. Not so and you can prove it.
Try it on a separate piece of stainless. Take 2 pieces and 5200 them together. Leave for 6 months to a year out in the wx and you will see that they do not hold at all.
I say this because my new boat (15 yrs ago) leaked at all chain plates and was just 6 months old. Later discovered hull/deck bolts leaking and all deck fittings.
3M5200 does not stick to stainless for long. Also Aluminum and bronze but a little better than SS. I have had to replace it everywhere with Silicone Sealer. Now that does stick and for a long time.
Glen Read

Mar 6, 2013 04:22 pm
 Posted by  glen

Another point. It is well known that Lexan crazes like mad after a few years in UV. All my hatches are Lewmar. Lexan was used and they are all badly crazed.

Mar 6, 2013 05:06 pm
 Posted by  Mushroom 39

I have used butyl rubber tape and gun grade butyl on my boat in the rainy NW for over 30 years with good results. The stuff I use to bed deck fittings is normally used for house windows, etc, and comes in narrower widths. It is naturally a bit sticky and tends to stick to the mounting bolts, which is good, except if you turn the bolts instead of the nut. Then it will ball up around the bolt stem. To ensure that the bolt hole is sealed I wrap a small strip around the bolt under the base, however, this means lowering the fitting into place with the bolts inserted in it. You need lots of hands!
Because it stays plastic, it tends to flow under pressure so it squeezes out. It's a good idea to have a small countersink (~1/16")around the bolt hole in the deck to form a seal ring.
If you use gun grade, make sure it 100% butyl rubber. The cheap stuff in the building supply places has filler in it and dries out over time. I bedded the main hatch storm cover with butyl tape 35 yrs ago and it is still pliable!

Mar 7, 2013 10:46 am
 Posted by  Priscilla

Thank you Dick for your article promoting butyl rubber tape! Butyl rubber tape (brt) on boats is not new - I learned about it in Eric Hiscock's 1950 book "Cruising Under Sail". Regardless of leaks, unlike bronze, stainless steel hardware and fasteners must be inspected for crevice and pitting corrosion, wastage and weld decay at least every five years, and renewed regularly (10 to 25 years) and brt makes that chore a little easier than modern sealants.

Some tips: Brt is oil soluble so do not use it for or near fuel fills, use a polysulfide sealant. Since brt compresses slowly over time and heat softens it you will need to snug down your fasteners several times. Remove squeeze-out by simply rolling it up in a ball. Brt can be easily and completely removed using mineral spirits. This tutorial shows how to properly bed hardware using brt:

Polyurethane (5200) is a "permanent" ADHESIVE - everything on a boat is a maintenance item and should be bedded with a flexible SEALANT. Silicone and silicone mix sealants are poor bedding choices because they leave a very difficult to remove "oily" residue on surfaces and nothing sticks to that residue, including more silicone. Although having said that, to date I have no leaks using brt in places where hardware was previously bedded with silicone.

Mar 7, 2013 11:42 am
 Posted by  Ocean Navigator

Thanks to OENGHUS, Glen, MUSHROOM 39 and Priscilla for the excellent comments.

Mar 7, 2013 11:50 am
 Posted by  smb2415

There is also White Butyl made which comes as wide as 2" and in various thicknesses. I have used 2" x1/4" in the past. As you are already aware the butyl also comes in Black and Grey. The white costs a little more but will be much less noticeable on the topsides. If you really want to create great long lasting seal using butyl there is a primer which would be applied to both sides of the material between where the butyl would be placed. The primer is extremely tacky, black, messy stuck but works wonders. Finding these items in small quantities may pose a challenge as they are usually sold by the case and pallets. FYI there is also a product similar to butyl sold in tubes ask the commerical glass installers about "sausage" (due to its packaging). I believe all these products are made by Dow Corning. I have used TB Philly ( Phone: 610-482-6001 in as a supplier for large QTY purchases, they may be able to help with smaller QTYs. I was going to post part# but their site is down.

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