Silicon Sealing underwater?

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steelfloats

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Can anyone offer some empirical based guidance on the use of silicone based sealants for through hull fittings? It seems to be the convention to not use it but go for the expensive chandlery supplied sealants. Is there proof of Silicones breaking down in Salt water, or is there another reason to refrain from there use under the waterline?
 
I could be way off the mark here but I think....

I used Sikaflex 291 for my skin fittings, on advice. But, it was good advice, the stuff is a nightmare to get off GRP, which is a good thing. Silicons peel away simply and do not instill confidence. This stuff bonds like a limpet, I would not use anything else now.
 
The only thing is that if you are ever likely to want to get the fitting off for any reason - do not use Sikaflex as its almost impossible to get a fitting off thats been bedded in this way. For fittings that may need removing at some time better to use silicone mastic. The trick here is to bed the fitting on silicone but to leave the edge of a washer or penny as a spacer under the fitting so it doesn't squeeze all the mastic out of the joint. When the silicone has cured, pull out the spacer and tighten up the fitting - that way you get a perfect seal every time
 
I had the same question in the pub a few weeks ago and went with the Sikaflex option.

I agree with Woofy on its use. Remove an item fitted with Sikaflex and compare it against one fitted with silicone and there is a world of difference. They are totally different substances - some one here will know the composition I'm sure.

The Sikaflex actually sticks to the components (And every other object within a square mile) , which gives me a lot of confidence in its sealing ability - the silicone peels away having formed as a seperate piece. After removing skin fittings done with both, I would never hesitate to use the proper stuff!
 
I tend to agree with you. The difference is that one is a sealant (and an adequate one in my humble opinion) and the other is an adhesive. I have had the misfortune to have had to remove skin fittings where Sikaflex has been used and it is difficult sometimes to do so without causing damage to the hull..
 
NASA issued a warning recently, published in PBO, that using Sikaflex to seal their skin fittings was causing some to crack !

Read it after I had bought a tube of the stuff for that very reason. /forums/images/graemlins/frown.gif
 
Hi Steelfloats!

FWIW, Raymarine, from whom I bought my new ST60 system, confirmed that Sikaflex 291 would be fine for their transducers. Hope they're right 'cos I've got a tube all ready to fit 'em!!! /forums/images/graemlins/confused.gif

Cheers Jerry
 
My Prout was fitted out with nothing but good quality marine silicone sealants when manufactured, and |I wouldnt believe the difficulty I had getting a Blakes seacock out of the hull. It not only stuck like sh*t to a blanket, but it turned into a rubber that even the angle grinder had difficulty with. So with a few exceptions where I used others out of curiosity / availability, I continued to use silicone under water without a single problem.

On the other hand, every time I tried Sikaflex, I was disappointed though the 3M equivalent was good. The one I really didnt like was polysulphide rubber.

Dont use bathroom silicone sealant, but IMHO there's nothing to beat marine grade silicone. And in my view Sika is one of those situations where something gets a name for being special when there isnt much evidence that it is, apart from its price. All IMHO of course.
 
1. That's as maybe, but Sikaflex is nothing to do with silicon. It's a polyurethane-based compound.

2. Anyone any idea how much Sikalflex differs from Evostick's 'Sticks like Sh*t' (their asterisk)? Both smell the same and leave the same godawful mess on your hands and everything else. Difference, of course, is price.
 
OK if you want to be pedantic! They are both sealants but Sikaflex also sticks like sh1t to a blanket and you can't get the bloody stuff to let go if you want to get the fitting out again. It is also very high modulus and can become very hard which is why NASA and others don't recommend it. I see no reason for a product with adhesive qualities anyway as fittings are screwed in place. A sealant is all you need and I have never known a good quality Silicone (not silicon thats rock) sealant fail underwater. Does that make sense to you now?
 
BoatMike - I wasn't being pedantic. I merely said that the point you were making regarding silicone being a sealant and silkaflex wasn't, was wrong.

You are of course, quite right that silicon and silicone are two different things. Silicone being a compound made from silicon. A crucial difference.
 
The only problems I have had with fittings letting water past have been with the few the previous owner has seated with some sort of silicone.
I have used Sika adhesive sealants for the last 4 years for a variety of purposes - staunchon bases, deck fittings, transducer through hulls (2 off), and rechaulking parts of a teak deck - different gunk, but they have all been 100% successful, and none have had to be revisited.
The Sika stuff is not so easy to use - but it is much better at what it does, so if you want to take it apart, or have it leak, use silicone, I guess!
I'll continue to use the real stuff, thanks
 
I fitted a new engine intake strainer, bedded on marine silicone, last winter. No problems so far.
When I first got my boat 10 years ago I did a temperory repair to the perspex sliding hatch. Despite not cleaning the parts (simple butt joint with minimal contact area) I have not been able to get the bloody thing apart again! Brian.
 
As others have said - Sikaflex bonds to the surfaces to be sealed, and forms a permanent seal. Permanent being very much the operative word. It flexibility allows for a degree of movement without releasing the seal it has formed. Fairly essential in most applications on boats where failure would start a leak.

Silicone must be allowed to cure and then compressed between the faces it is sealing, forming a compression gasket which is highly effective and easily dismantled later on.

If it is compressed before it has fully cured it is simply pushed out of the joint. The film that is left is not thick enough to compensate for any movement or flexing of the joint, and will not remain sealed for long.
 
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