Sealant: PU/Silicone/Butyl etc. use cases

laika

laika

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The guys who showed me the basics of boat maintenance used a pu sealant for everything and I've pretty much done the same ever since. Recently I had a chat with an old mate I hadn't seen for years. Very smart guy, ex superyacht skipper and now runs a marina. He uses silicone sealant for just about anything that doesn't require lots of adhesion.

It strikes me that sealants give you 3 main things. Sealing against water ingress, cushioning and adhesion. and maybe some thread locking. Mostly I'm not interested in adhesion: I want to stop water dripping down the threads of deck fittings for example, not glue them in the holes: there's nuts for that (or am I overlooking load spreading because of the adhesion?).

So when to use silicone vs. PU sealant (or butyl etc.)? I'm wondering about general guidelines and certainly not avocation of people's favourite brands. I'm not so worried about cost (within reason) . A tube of sikaflex lasts a long time and from my local force 4 costs less than any builder's sealant once you've factored in bus fare to a builders' merchant and back.
 
Breaks into a few groups.

Silicone sealant in my experience is flawed & seldom reliable (can cause corrosion on alloy), although neutral cure silcon is better (the one that DOES not smell of acetic acid) as it does adhere better and is tougher Low - expensive depending who sells it to you.

P/u sealants/adhesives can be impossible to break the seal on removable items if ever required without damaging the substrate, so only use where almost permanent mechanical security is important.

Oil based sealants & mastics are pretty universal for keeping water out above & below the waterline without causing damage if the area is later stripped.(Window frames etc Quite low in price).

Acrylic sealants only above waterline and only apply if there is no chance of rain for a few hours as it all washes away in rain before it is cured Again very low in price.

Butyl rubber generally good for almost any application depending on the manufacturer but difficult to find at moderate price.Tough & waterproof, bit difficult to squeeze out on perspex windows sometimes as it can be a bit stiff.

Newer MS polymers are very good, can form a structural bond & seal so can be difficult to disassemble, but does not go off in the tube like Sikaflex & other P/U sealants do. Moderate in price and a lot of manufacturers in differing colours. Also find use in deck caulking now as it is said to be U/V proof with very good adhering qualities.

These are only in my experience of the main sealants in use, I am certain that other users have differing views.

John Lilley
 
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johnlilley said:
Butyl rubber generally good for almost any application depending on the manufacturer but difficult to find at moderate price.Tough & waterproof, bit difficult to squeeze out on perspex windows sometimes as it can be a bit stiff.
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Arbomast BR is available from Marine Superstore, Port Solent, at £4.90/cartridge and from Arthur's Chandlery, Gosport, at £4.85/cartridge.

I have been using it recently for bedding some deck fittings. It's easy to use but it shrinks a lot, leaving a gap between the fitting and the decks which has to be filled with more compound applied with a cartridge gun. It is also very soft and easily damaged.
 
parsifal said:
Arbomast BR is available from Marine Superstore, Port Solent, at £4.90/cartridge and from Arthur's Chandlery, Gosport, at £4.85/cartridge.

I have been using it recently for bedding some deck fittings. It's easy to use but it shrinks a lot, leaving a gap between the fitting and the decks which has to be filled with more compound applied with a cartridge gun. It is also very soft and easily damaged.
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Dont you use temporary spacers under the fitting so that it can later be pulled down after the spacers are removed .

Trying to fill a gap sounds very unsatisfactory.
 
VicS said:
Dont you use temporary spacers under the fitting so that it can later be pulled down after the spacers are removed .

Trying to fill a gap sounds very unsatisfactory.
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I did what the manufacturer recommended!

Question to the technical department: Would you advise inserting spacers between the mating surfaces to ensure a minimum thickness of sealant (eg nylon washers)? Or is it better to simply tighten down and squeeze out the surplus sealant?

Answer: Regarding spacers, I don’t believe this is necessary as any thickness of BR will create a seal

:(
 
Where anyone has used silicone it will be hard/impossible to use any other kind of stuff that needs to stick to the boat. If you have a boat that needs paint or varnish every now and then, better to never use anything that has a hint of silicone.
 
It's not just the choice of sealant but the way the job is done. For example, squirting sealant around a deck fitting that is causing a leak down below is not nearly as good as taking it off and rebedding it properly with new sealant in the through deck part, even though the latter is a good bit more of a hassle to do.

Also, sometimes people use sealant when epoxy / resin would be better.
 
If it turns out that the silicone does not do the job, all you are left with is fixing stuff mechanically. No glue/sealant will ever hold as silicone is impossible to remove.

And that also means impossible to sand away.
 
SvenH said:
If it turns out that the silicone does not do the job, all you are left with is fixing stuff mechanically. No glue/sealant will ever hold as silicone is impossible to remove.

And that also means impossible to sand away.
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That's not my experience. I removed all my windows for refurbishment and they had a mix of sealants applied over the years including silicon. It just scrapes off GRP and a good scrub with acetone and wire wool preps the GRP surface for new sealant. The inside of the window has ply lining which was plastered with sealants, again this included silicon. Scraping and then sanding brought the wood up again for revarnishing which was not an issue. I agree that a silicon lubricant spray that soaked into wood would be an issue, but not silicon sealant.
 
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