There I was, sitting fat, dumb and happy. I had resolved my sealant issues years before. I could recite the mantra by heart. Seal the deck, mask the deck, apply a rubber washer under the fastener, apply 3M101 above the waterline or 3M5200 below the waterline, and tighten the fastener down on the rubber washer. I had it down pat. Then 3M stopped making 3M101 sealant and threw my world into disarray. Did I switch to 3M 4000 or 3M4200? What about all the SikaFlex sealants and their mysterious numbers? What to do, what to do?
Well, maybe it wasn’t quite so dramatic, but 3M did stop making my favorite above-the-waterline sealant, 3M101. It was a polysulfide sealant that wasn’t a tenacious an adhesive as 3M5200 so fittings could be easily rebedded when necessary.
Along about that same time I had occasion to remove a thru-hull fitting from one of my project boats. That thru-hull had been installed by some good old boy down in Chesapeake City when the boat was built in 1978. I was sure it was bonded permanently in place and would take a great deal of force to break it free. I assembled my tools: a thru-hull removal tool that fit the lugs inside the thru-hull, a pipe wrench to fit the tool and a four-foot piece of pipe to fit the pipe wrench handle to act as a breaker bar.
To my surprise, all that hardware was unnecessary. While the fitting was completely bedded and well-sealed (it had never leaked a drop), it unscrewed relatively easily. The fitting had been bedded in a sticky black goo that remained flexible and sealed for all those years.
A little research on the ‘net turned up a sealant from bygone days, butyl rubber. It is still being used as a construction adhesive and in tape form for things like car windshields and RV windows but had fallen into disuse for boats. Still more research turned up a marine web site featuring a host of DIY projects, including the use of butyl rubber tape:
Not only did the site cover using butyl rubber tape, they had also sourced a supply for marine grade tape and offer it for sale. One thing I had discovered in my research is that there are many types of butyl tape available and not all are good enough for use aboard boats. They are available in a wide variety of sizes, colors and hardness. Most RV stores, for example, will have some form of tape available; but I chose to buy mine from a marine source I felt I could trust.
That tape comes in the form of a roll of tape, about 1/16’ thick and about 5/8” wide. The layers of tape are kept separated by paper as the tape would stick to itself otherwise.
This form of butyl rubber is for use above the waterline. While my thru-hull was bedded in butyl rubber, I believe it was applied from an applicator or tube, not in the tape form. Butyl tape is not an adhesive, so I wouldn’t use the tape below the waterline.
The tape is also harder to apply and use in colder weather as it is stiffer and less sticky.
So, how do we use butyl tape to rebed deck fittings? Well, the first step, as in all rebedding projects, is to seal the deck. That means cutting out the core around the fastener and replacing it with epoxy filler. The mounting hole is then redrilled through the epoxy plug. That epoxy plug prevents any future leaks from getting into the core and compromising it.
Some have suggested using a bent nail in a drill to hog out the core but I find the nail bends and breaks in all but the weakest core material. I have had better luck using an Allen key. Shorten the short leg of the key and sharpen it to act as a cutting edge. The hardened Allen keys will stand up to harder cores, like plywood, much better than a bent nail.
Vacuum out the debris and fill the hole with epoxy filler. Remember to tape the lower hole or you will have an epoxy stalactite hanging from your overhead. Redrill the fastener hole. In the past, several people have suggested cutting a slight countersink in the top of the fastener hole and then using an O-ring on the fastener. The idea is that the O-ring will wedge in the countersink and help seal the fastener. The problem is getting the countersink just the right size so the O-ring does wedge properly without being too loose. With butyl tape, you can use the countersink idea but it is much less exacting.
The next step is to cover the bottom of the fitting with the butyl tape. Butt sections of tape together to completely cover any mounting surfaces. Try and stretch the tape as little as possible as you apply it to the fitting. You can trim the edges of the tape to fit the mounting with a sharp knife or even a pair of scissors. Use the fastener to punch holes through the tape where the fasteners will go. Cut a small piece of tape and roll it into a round shape, about the diameter of an O-ring. Place it under the head of the fastener and then push the fastener in place. Roll a similar piece of tape and place it around the fastener where it exits the fitting. This bit of tape will wedge into the countersink around the mounting hole, further sealing the fastener.
Now you can bolt the fitting in place. Tighten the fasteners slowly and uniformly. It is critical to not allow the fasteners to rotate; hold them in place and tightened the nuts from below. Allowing the fasteners to turn will negate any seal around the fasteners and speed leaks and sealant failure.
Any small amount of squeeze-out can easily be cleaned up with a putty knife. The tape nature of the butyl means you have no need to extensively tape off around fittings or do extensive cleanup of messy sealants as in 3M4000, 3M4200 or 3M5200.
I mentioned the cold weather limitations of butyl tape above. In addition, butyl tape will tend to creep somewhat in tropical weather. The exposed edges of the sealant remain tacky and could attract dust and dirt. Petroleum products will dissolve butyl tape so it isn’t recommended around fuel fills and similar fittings.
On the positive side, the application is so much easier and less messy than typical marine sealants. Butyl tape also stays flexible and sticky through the life of the rebedding process. If I can’t have my 3M101, give me my butyl tape, please!