Silicone; Dont use it and why.

[CharlieJ]

One huge problem with silicone is the residue it leaves( among other problems). Should you ever try to paint over where it was (or where it got by accident) you'll find out why those of us who get paid to work on boats HATE the stuff.

We're now switching to Butyl rubber. Got a caulking tube  of it from Ace hardware. Re bedding companionway trim with it tomorrow

[phil416]

    Charlie: Amen!  I have switched to 3M strip calk ( available at car parts stores) and carry a package aboard at all times. This stuff cleans up easily with mineral spirits Or even diesel fuel in an emergency.   Fair winds Phil

[s/v Faith]

I have been very happy with my silicone sealed portights. 1/2" plexi with a RTV (room tempurature vulcanizing what ever that tells you) black gasket silicone. The main thing with them though is that the silicone acts as a gasket not an adhesive, the windows are screwed in with stainless screws and I let the silicone cure before tightening the windows down. It stays soft even in the sun, most other products like sikaflex or 5200 will harden and eventually the window after some unpreventable twisting will leak, but the silicone is still as soft and rubbery as the day it first cured.

Silicone can function as a compressive gasket.  It does not function well as a sealant.

The problem comes in where it is allowed to come in contact with wood or fiberglass.  The silicone oil
seeps into the gellcoat, and even into the fiberglass it's self.

Paint will not stick to it, neither will epoxy.    Some make the error of thinking that automotive solutions will work for fiberglass... in automotive prep work, silicone is a big problem because of silicone based waxes.   The difference there is that the silicone is MUCH easier to remove from the metal surface then it is from fiberglass.

Many have suffered as I have with this problem...  I have had to actually chisel out sections of fiberglass after experiencing epoxy and paint failures where someone had slathered silicone sealant onto a surface to 'stop a leak'.

[haidan]

Ah, interesting didn't know about the gelcoat thing, I'm using it on painted steel, and the black silicone makes a nice visual framing for the window so no need to paint it

[roverhi]

Sillycone belongs in boobs not boats!!!  Actually seeing the hard bumpers on so many women these days, don't even think it works on boobs.

My first boat had all the hardware bedded in Sillycone from the factory.  Had to R&R every piece ofhardware and rebed with LifeCaulk because all the fittings leaked like a sieve.   My current boat was riddled with SPOT sillycone everywhere above and below the water line.  I've completely stripped every peice of hardware off the boat to be sure that I banished the poop.  About the only good I can say about sillycone is it's easy to remove hardware because it doesn't stick to anything except what you don't want it to stick to.  Getting the globs of it out of screw holes and the port surrounds was a pain.

LifeCaulk does not harden with age.  I've pulled up stuff I bedded 30 years ago and it still adhered to the surfaces and remained pliable.  LifeCaulk (polsulfide) doesn't shrink so no need to go back and cinch down fasteners later.  Both 5200/4200 and LifeCaulk actually require moisture to cure.  Out of the water, they pull it from the air.  Because of the need for water to cure, they work just as well when applied underwater so great for repairs.  LifeCaulk doesn't seem to harden in the tubes as quickly as the polyurethan sealants do.

Certain plastics like Beckson ports will crack if sealed with 5200/4200 (polyurethane) or LifeCaulk/101 (polysulfide).  To cure this need for sillycone I won't use anything that is not compatible with the 'Poly' sealants.  Not fond of the way the Beckson ports look anyway. 

If You have something to seal that will have lots of thermal expansion caused movement like large portlights, use Butyl.  The only large port in my boat that didn't leak was sealed with Butyl.  The sillycone sealed ones all leaked but were real easy to take out.