March 15, 2016

Tried and trusted sealing compounds

IT SEEMS THAT more and more of us are using butyl tape to seal our deck fittings. Butyl has been around a long time and it’s very useful, especially for sealing car windshields. But it has no adhesive powers. Which means that a wobbly stanchion base is going to let water in after a few not-so-dainty crewmembers have lurched against the lifelines.

What you need here is a flexible sealant that sticks tightly to the bottom of the stanchion base as well as to the deck, something that can stretch temporarily without opening up a gap for water to seep through.

Don Casey the man who wrote This Old Boat, the boat repairer’s bible, says there are three main bedding sealants:
Polysulphide is what he’d use for deck fittings. It’s available in single and double packs. Twin packs cure more quickly. Use it for all kinds of sealing and bedding except for plastics. It will melt plastics. Polysulphide remains pliant and adheres very nicely to each side of the joint, although you can remove the fitting without too much trouble when necessary. It’s good for caulking wooden deck seams, and you can paint or varnish over it.
Polyurethane is another good sealant, but it’s also a very strong glue. So use it for permanent joints only. Don’t use urethane on fittings you might want to remove later. Don’t use it on plastics such as Lexan or ABS, either. You might be able to paint over some polyurethanes, but not most of them. Check the instructions.
Silicone is a good bedding compound and sealant that you can use on almost anything, including plastics. Most silicones are not particularly good adhesives. Better than butyl, though. Some new silicone formulations have better sticking power and might be difficult to remove at some later stage. It makes good gaskets, but you can’t paint or varnish over it.
So there you have it. If you trust Mr Casey, as you should, your course is clear. Ignore the herd. Forget the butyl. Grab the polysulphide.
Today’s Thought

An expert is somebody who is more than 50 miles from home, has no responsibility for implementing the advice he gives, and shows slides.
—Edwin Meese 3rd
“What’s the penalty for bigamy in Utah?”

“Multiple mothers-in-law.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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