Sealant

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pbeard

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I have just removed both cockpit drain seacocks and will obviously need to seal them on refit. Could anyone recommend a sealer please, the stuff on them originally was a mauvey colour and had stayed fairly soft over the years - which made the removal fairly straightforward.
 
If you don't want to pay the marine surcharge you can use any Polyurethane sealant adhesive from you local builders merchant.
 
At the advice of a local shipright, I use plain old building mastic for jobs such as yours, where a watertight but easily breakable seal is required, and where the fitting does not rely on the sealant for mechanical strength. It's dead cheap from builders' merchants (£1.50 for 380cc for the last one I bought, made by Sovereign, by appointment to Liz Windsor. Bet she doesn't use it on a boat, though). The cartridge doesn't say what's in it, but it seems to be oil-based, never goes hard and sticks like the proverbial. Comes in grey, white, brown and possibly other colours.
 
I really dont understand this obsession people have with polyurethane adhesives. The title of the post of 'Sealant'(not adhesive) and pbeard quoted staying soft and ease of removal as his prefered criteria. Yet people start recommending polyurethanes???

Polyurethane will stick so hard it will be more part of the boat than anything else. On gelcoat polyurethane can reach up to 500psi of adhesion, that is far more than the gelcoat is adhered to the matting. Polysulphides, on the other hand, will form just as good a seal, but only at about 50psi so it is possible to remove it.

I'm very interested to know why people are recommending polyurethane over polysulhpide sealants for this application?
 
Sikaflex 291 is a polyurethane. Ok for your job. You could use a silicone rubber; it has low adhesion, high cohesion i.e. it makes a pad which will easily peel off.
 
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Simply becouse they dont have a clue!!!

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OK you may be very knowledgeable but as yet you have not offered YOUR advice!

I advised Sikaflex 291 a) because it is what our local yards/pros use and b) it is easily available. It is an adhesive but not really aggresive (250psi 'ish) and that might be an advantage for sealing a skin fitting that will not be removed probably for many many years. It's associated seacock however might need changing and that will be easier with the skin fitting held and not turning in the hole. If the seacocks are Blakes type and bolted through it might be different. 3M's products I'm sure are fine too, just not so easily found around where I am.

Personally I wouldn't use a cheapy builder's merchant sealant below the waterline either, a few pennies saved might cost a few more later, why risk it? Above the waterline is different, correcting a problem doesn't require a lift out.
 
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It was suggested not because it's cheap, but because because it works.

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I'm not saying it doesn't work, just that personally I wouldn't consider using something that wasn't designed and sold for underwater use on MY boat unless I was very sure it would work and continue to work for many years. It follows that I wouldn't recommend someone else to use what I would not use myself. Just my personal paranoia maybe.
 
Totally agree here.


Also 3M marine products such as 101 are hard to find but if you speak to 3M they will telly ou who there local distributor is.

Dom
 
There is quite a lot of misunderstanding where sealants are concerned. Polyurethane can be both a sealant and an adhesive/sealant. The former are lower modulus and more flexible. You cannot have very high modulous adhhesive type without sacrificing stretch or the ability to stretch in a joint which is why low mod silicone is used on upvc windows and high mod around wash basins etc. The soft mastic that doesnt harden is fine underwater as long as you have mechanical means to hold the pieces together. These types are generally called Mastics and common types are Arbomast, Evomast and Secomastic. They are at the bottom of the sealant chain and are basicaly slate or chalk dust with veg oil . A very similar product is made with butyl and also never hardens until it has broken down which is caused by the air thickening the skin as the months go by until the thicknes of the material is total crust and then fallsl off.Overpainting can slow the process down while underwater it will remain stable as it can`t dry out. So these types can only be used on a boat where no air will get to them. Ordinary builders type Polyurethane can be used anywhere on the boat and can be cut out with sharp blades no problem. Building type Polysulphides likewise and both these materials can be overpainted and cure to a nice flexible rubber. Silicones are probably the most durable of all, I think low mod is preferable as it doesn`t give off the acid which high mod does and this is not good for grp. Silicone may not be as good on wood and also it can`t be painted. If doing critical joints with any material Primer is a good idea especially on wood or anything that isn`t stable. The primer produces a glass like surface when dry which is when you should gun your material on. Needless to say stable surfaces like sound plastic or glass etc needs only degreasing and no benefit is gained by primer despite what someone may try to sell you. Most primer contains Isocynates so be careful in enclosed spaces.
Cheers Alex
 
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