Discussion in 'Materials' started by Frosty, Jan 4, 2009.

  1. Frosty

    Frosty Previous Member

    Gasp horror, the lewmar site suggests silicone as a bedding compound for its deck hatches. This cant be right --can it?

    I am about to replace the sealant on some deck hatches. They are not leaking but the sealant looks a bit gruby so I thought I would do it while the sun is out.

    I bedded them in with Sikaflex 5 years ago and I can tell already they will be a ***** to get out. However back to the silicone that cant be right can it?

    What should you use?
  2. Guest62110524

    Guest62110524 Previous Member

    sika 11 fc or some v expensive 3m ****
    look these kids in the Lewmar office, wot they know? cant even paint silocon, and now sika have a uv product cos I had some failures in my Qld boats after 7 years there was some breakdown of the sika
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    There is nothing wrong with that suggestion, but they probably mean the silicone compound for marine use, without the smell of vinegar. Moisture in the air does the curing process, the end product is much more flexible than polyurethane (Sika) and absolutely impervious to UV.
  4. divinesd
    Joined: Jan 2009
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    Location: Divine service & design, Rotterdam, Holland

    divinesd Divine service & design

    UV can do terrible damage to the bedding of hatches (2-3yrs) when using a 1 pack polyurethane product like Sika. I remember Rondal advized a silicone based product as well. These specific silicone based products seem to have a better resistant to UV and salt. It might cost a bit more, so do check after curing, with water, for any leaks.
  5. Frosty

    Frosty Previous Member

    Well what about Sikaflex 295 UV ? I dont think I can get silicone UV marine that does'nt smell of vinigar,--I do know what you mean though.
  6. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    I think CDK is talking about what's known as a "neutral cure" silicone. You would probably want a low modulus, UV resistant type for hatch bedding. Dow, 3M, Tremco and GE all make products that might be suitable. I don't think it's necessary for it to say "marine grade" on the tube- just go by the mechanical and chemical properties. A low modulus neutral cure silicone intended for exterior use with acrylic or polycarbonate should be fine, boat grade or not. (Dow in particular is notorious for selling the exact same product under two or three different names, at different prices, the only difference being tech support and warranty.)

    I don't see any reason why neutral cure silicone would be a bad choice here; the acetoxy types (that reek of vinegar) might attack the clear plastic of the hatch. The trick is getting them to bond- scrubbing the substrate with isopropanol, then drying it, can help. Once bonded, the silicone should outlive the boat.

    Sikaflex 295 is specifically designed for bedding acrylic and polycarbonate windows and hatches. It is a moisture cure polyurethane; I don't know how long-lived it would be.
  7. divinesd
    Joined: Jan 2009
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    Location: Divine service & design, Rotterdam, Holland

    divinesd Divine service & design

    Silicone or??

    Personally I have bad experience with the adhesion of the 295. The 291 was even better but can't resist UV. Therefore we recommend ARBOSIL 1081.
    I have looked up the pds. See attachment.

    Attached Files:

  8. Frosty

    Frosty Previous Member

    Its an alluminium hatch suround not plastic windows.

    I am very limited to what I can get, but I have just come from town and I did see some roof repair silicone and it even suggested it was for boats. Its only 2 dollar a tube!! I cant get 3M or any of the others mentioned but sikaflex is available everywhere.
  9. Meanz Beanz
    Joined: Jun 2007
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    Meanz Beanz Boom Doom Gloom Boom

    If the Sika ain't broke why touch it! It will be a ***** to get out... then if you do why not Sika again, best sealant that I have used.
  10. Frosty

    Frosty Previous Member

    On the lewmar site it shows a real nifty way of using a G clamp to pull the frames out. Ive made some blocks of wood ready to have a go but I think they will really be a *****--*****. One window did show signs of a leak so I just gunned around it and cured the leak promising myself I would do all of them when the sun shone again which is now.

    The biggest job will be removal. I am reluctant to use Sika again because of the "***** factor".
  11. goodwilltoall
    Joined: Jul 2010
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    goodwilltoall Senior Member

    Was looking up advice on installing ports since i have two 7" ports at engine location that will be put in shortly.

    For glass, silicone is one of the best materials to use and as cdk mentioned the cheapo vinegar smelling tub surround silicone is the one to avoid.

    Dow makes the 795 structural sealant which is used on high rise glass facades, $11 for 20oz with a few colors available, they have one lil better, called 995 which is only available in black, $13 for 20oz. Plus no primer neccesary.

    Structural silicones are about the only materials that bond to glass, when replacing you actually have to grind it off with a diamond abrasive to get completely off vs. the ureathanes which can be cut with a sharp blade. If new clean with alcohol

  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    For acrylic, polycarbonate and glass, use silicone, for aluminum and other materials, use UV resistant polyurethane.
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