April 26, 2009

The battle of the polys

THERE’S ALWAYS SOMEONE who knows better than the experts. My friend Sam Psmythe (silent P, as in bath) is one such man. He read on some bulletin board that the best sealant for deck fittings is butyl rubber. Despite my protests, he is going to re-bed all his lifeline stanchions in butyl rubber tape, gawd help us.

Now butyl rubber is actually polyisobutylene, very good stuff when you use it in the right applications. It would make a good sealant if the stanchion base didn’t move relative to the deck. But you know what people are like. Always flopping around on deck, falling against the lifelines and straining the stanchion bases. Now, butyl tape lacks the necessary adhesive power, so, as soon as a gap appears between the base and the deck, water will gallop in and trouble will soon follow it.

What you need for most deck fittings is a bedding that not only remains flexible, but also has tenacious adhesive qualities, so that it sticks to both the stanchion base and the deck like sh*t to a blanket, as those rude Australians say. Then, as the base moves under stain, the bedding sealant simply stretches momentarily without letting any water underneath.

The expert I have in mind in these matters is an acquaintance of mine called Don Casey, the man who wrote This Old Boat, the boat repairer’s bible.

Here are the three main bedding sealants according to Casey:

►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.
►Silcone 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 more than Mr. Psmythe, as you should, your course is clear. 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.”

1 comment:

Anonymous said...

You might want to consider some other "experts" aside from Don Casey -- top-notch boatbuilders like Hinckley, Morris, Cape Cod Shipbuilding (who built my Dad's Mercer 44 back in the 60s), Whitby Yachts (which built my Alberg 37) -- all of whom have used butyl rubber for bedding stuff. Then there's also a whole industry -- roofing -- that uses the stuff all the time, in applications where motion of the base-surfaces (e.g. a wooden house) is common, and use-life is something like 20-30 years typically.

I've bedded things on my boats over the years with 5200 (a huge mistake -- tore out gelcoat when I had to remove it because it didn't work), silicone (doesn't stick well, but leaves slimy residue that can only be removed by abrasives, which spread it around!) and even polysulfide, which I mostly love, but which failed within two years on my gate-stanchions, despite careful prep and installation. The butyl rubber installation, however, is holding up fine, and the nav desk is dry as a bone.

So ... while you're certain, because Don says so, that one of three bad choices is best, I'm certain, because of my experience, that a fourth is better.

Your readers might want to take a peek at this .