When shopping for a used sailboat, one of the most important things to look for is soft or springy spots in the deck. They’re a sure sign that water has seeped into the wooden core and caused rot that’s difficult and expensive to repair. Water damage is also common around the edges of wooden bulkheads, chain plate bolts, and anywhere else that hardware penetrates the exterior fiberglass. There is no easy fix for water damage but preventing it in the first place is quite simple if you seal all penetrations with butyl tape.
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Butyl is a pliable, synthetic rubber compound that does not harden with age. Butyl tape is a rolled strip of thin butyl that is a convenient way to store, measure, and use butyl as a marine sealant. Bedding hardware is the process of installing the hardware in a “bed” of sealant to keep water and grime out.
Butyl tape is particularly good when compared to other marine sealants for a bucket full of reasons:
- No smearing
- No running
- No cleanup
- No waiting to cure
- No hardening in the tube
It’s inexpensive and is even reusable in some cases–try that with 3M 4200 or 5200! Those sealants are also good and are best used in specific applications but for general ease of use and future disassembly, nothing beats butyl tape.
There are right ways and wrong ways to apply butyl tape. What follows is the procedure that I learned when I got started and it has served me well ever since.
Make your bed and sail in it too
To bed hardware with butyl tape:
1. For new hardware, drill a normal size hole to receive the fastener. For straighter holes, see DIY Drill Guide for Accurate Holes.
2. Chamfer the outside of the hole with a countersink bit or a larger regular drill bit. The general rule of thumb is twice the diameter of the holes. If you’re using hollow finish washers like I show here, the chamfer can be minimal because the washer itself will hold plenty of butyl.
This will do two things:
- Remove microscopic cracks in the gelcoat that can spread when the fastener is under pressure.
- Make a funnel around the fastener for the butyl to squeeze into and make a more reliable seal.
3. Tear a piece of butyl tape from the roll long enough to seal the hole when rolled into a string and shaped into a ring. For most small fasteners, this is about the size of a Cheerio.
4. Knead the butyl with your fingers until it is as soft as it will get, especially in cold temperatures. It should be the consistency of soft clay. Then roll the butyl between your fingers into a string 1/8″ to 1/4″ in diameter, depending on the size of the fastener and the gap to fill. For sealing screws, bend the string into a ring shape just large enough to slide onto the fastener. Roll the ends together until they join completely. For larger or irregular shaped holes, make a shape that completely surrounds the hole.
5. Slide the butyl ring onto the fastener and tight against the washer or head. For finish washers like I show here, the butyl should easily fill the washer with excess to spare.
6. Insert the fastener into the hole and press hard to compress the butyl and force it into the chamfer.
7. Tighten all fasteners as usual. The excess butyl should squeeze out around the fastener, a good sign that it is also squeezing into the chamfer and fastener threads.
8. Even though it seems soft enough, the butyl will continue to squeeze out for hours or even days, depending on the amount and the air temperature. Retighten any loose fasteners the next day.
9. When the butyl has finished squeezing out, carefully trim off the excess with a putty knife.
If you want to get started bedding your hardware with butyl tape, I recommend that you begin with one 3/4″x30′ roll. That’s enough to seal practically every hole in a small sailboat but you’ll probably find other uses for it too, like RVs, home weatherproofing, etc.
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The $tingy Sailor 
I have stopped some deck leaks. Next step on our 1979 Catalina 27 is to pressurize the hull and find elusive water leaks.
Thanks for your articles, Stingy.
Oh! We recently saved our cushion fabric covers that came with fiberglass dust contamination! Two gentle cycle washes in big laundromat machines with liquid Tide and Oxyclean followed by air drying. Came out beautiful! Vacuum trick made install sweet. Thanks again.
Great advice! Thank you. Please note that not all butyl tape is made to marine grade quality. I tried re-bedding my port lights on my C-22 with bargain-priced butyl tape and as soon as the sun started to work it, the butyl melted, lost it’s seal and made a mess. Also, as a woodworker, I know that I get a cleaner, smoother chamfer by using the chamfer bit first and then drilling the hole. That would work fine for an initial bedding process anyway.
Good tips, Rocksterr
If you find the brand of that butyl tape, it would be good to pass along to help others avoid bad results. I purchased my last roll from Sailrite and it works fine.
Wow! I had no idea this stuff was around. Seems to make bedding hardware a lot easier than using 3M 5200, which ended up being a MESS! What is the orange hand drill you are using to chamfer the drill holes?
Hi, Allan
It’s a $5 countersink tool from Harbor Freight that comes with 3 sizes of bits. Quite handy.
Besides being messy, parts you assemble with 5200 can be extremely difficult to disassemble. It’s a very strong adhesive. The previous owner of my boat used it to seal the weather board to the sliding hatch. I nearly broke the board removing it and wood tore out in places instead of the sealant. So I only use it where nobody will ever have to take it apart.
Stingy,
About to re-install my refinished exterior teak And wondered about the best way to seal it. Would you recommend this take for that application Too?
Thanks,
John
Sent from my iPhone
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Hey, John
Yes. For the wider parts, I roll the butyl tape into long strings 1/8″-1/4″ in diameter and apply them as a gasket around the edges. For narrower parts, like the hatch glides, I apply a single 1/4″ string down the middle of the part (and around screw holes) that squeezes out to the full width.
Stingy,
Your blog post was most timely for me. I am about to remove and rebed all the deck hardware and stanchions on my 1985 Mac25.
You provide a great service to those of us with small pockets but often large dreams!
Hi there. Your post brought back memories from last summer! My teenage son and I re-bedded the stanchions on both sides of our 1975 Catalina 22. We never could have managed this work with any other type of sealant!! We also drilled out and re-potted the holes with epoxy. That did get a little messy because we are beginners. From my research I found that the best butyl tape comes from Compass Marine: https://shop.marinehowto.com/t/bed-it-tape. Prompt service too. He also has excellent how-to photo essay on bedding marine hardware and re-potting holes with epoxy.
This coming season we will tackle re-bedding the stays, the pulpit rail, and other hardware that pierce the deck.
I have a question about washers. Why use a hollow one? What are they for?
Thanks!
PS. This is an instance where getting the best quality has to be worth the price. I cannot imagine having to do this work over again due to cheaping out on the butyl tape!!
Hi, Rachel
Finish (hollow) washers just look a little more, well, finished than flat washers but they also hold a lot more sealant.
Ever since I had to replace a bunch of balsa core on an older boat, I’ve been paranoid about any water getting into a core. So I do use butyl tape like this, but as another person said, I also first drill a larger hole, tape it from underneath, and fill that larger hold with thickened epoxy. Once cured, I drill the appropriate size hole and use a countersink bit and then do the butyl tape. Works great! Not sure the epoxy step is worth doing, but it gives me peace of mind and it’s easy.
Hi, Aran
It’s certainly worth it if you’re doing a complete refitting, in areas with damaged deck core, or for larger holes that are more susceptible to intrusion.
Thanks for your comment,
$tingy
Have started using butyl tape this season on an old Catalina 36 where every thing is leaking. It will be hard pressed for 5200 or 4200 to find a spot on my boat ever again.
Log onto Marinehowto.com for more great info like this site and to learn about butyl tape. They formulated their own tape and sell it.