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post #1 of 10 Old 11-10-2009 Thread Starter
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plexus structural adhesive

i am currently redoing my hull deck joint (orignally 2" inward flange, through bolted 1/4" bolts on 3 inch centers with non-setting compound sealant) on my 37' boat.

i was wondering if anyone here has used this type of adhesive for such a purpose, and if so, how did it turn out. the specs look good. http://www.itwplexus.com/UserFiles/File/tds/30.pdf

tom
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post #2 of 10 Old 11-10-2009
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I would be hesitant to use that stuff on the hull deck joint for several reasons. First, I don't know how elastic it is. Second, the bond strength is far too high, and will become a problem if you need to disassemble the hull deck join for repair in the future. It is significantly stronger than the gelcoat to fiberglass bond IIRC. Third, I don't know if the material has a long enough working time to be really usable in this application.

Given that your hull deck joint is through bolted every 3 inches with 1/4" bolts, you may not really require an adhesive sealant for the joint and might be able to use a much more user friendly sealant, like butyl tape instead of a polyurethane-based adhesive sealant, like 4200 or 5200.

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post #3 of 10 Old 11-10-2009
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Both my boat, a 1977 CS27, and Maine Sail's CS36t were built with butyl to seal the hull/deck join and it's both not leaking today and still flexible. CS used it on almost if not all above the waterline fittings and it's an excellent choice. Also the least expensive. See Maine Sail's link: Re-Bedding Hardware Photo Gallery by Compass Marine at pbase.com
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i am aware of the butyl tape/caulk option. i believe this is what was in the joint to begin with...some sort of non-hardening compound, as well as some "spot" repairs in a few streatches of presumably polyurethane. however, this has been leaking for some time 70's boat also).

i ask because new products come out. J boats is using this on their new boats, among others. it has both high strength and flexation. i know others use sitaflex or 5200, but i don't think i would go that route. and yes, the product would supossedly be a permanent solution or nearly so, as in impossible to get apart without huge damage to other pieces.

i guess the real question is somewhat related to hard joint vs flex joint. perhaps a hard joint needs a greater surface area. this would make sense to me.

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If the bolts are used to mechanically hold the hull to the deck as they are, the sealant doesn't have to be "structural". The goal of the sealant is to remain flexible and keep out water. Butyl has the most elongation of any sealant I believe and meets the above needs very effectively. I wouldn't use 5200.
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Quote:
Originally Posted by sassafrass View Post

i ask because new products come out. J boats is using this on their new boats, among others. it has both high strength and flexation. i know others use sitaflex or 5200, but i don't think i would go that route. and yes, the product would supossedly be a permanent solution or nearly so, as in impossible to get apart without huge damage to other pieces.

i guess the real question is somewhat related to hard joint vs flex joint. perhaps a hard joint needs a greater surface area. this would make sense to me.

tom
Most boat builders today use Plexus adhesives but they make a number of them. The stuff you gave a link to is usually what they use for grid pans and liners to adhere them to the hull and it has quite low elongation when compared to other products.

None of this stuff will work well if the two mating surfaces are not 100% clean. This will be nearly impossible with an inward flange. You can clean the exterior overlap edge and lay a bead of Sika 291 or 3M 4200 along it then let it cure and re-bed the toe rail over it.

Short of lifting the deck off the hull by 6" to clean it and re-bed it any attempt at re-sealing, short of fiberglass cloth and epoxy, will be a "cross your fingers approach". Some builders used Dolfinite bedding compound which is closer to plumbers putty and not really thick enough or aggressive enough for a hull/deck..

It may just be the bolts leaking and you could always remove them one at a time and re-bed them too.

At least yours can come apart if you have to. My buddies boat was sealed with 5200, and is leaking, so he's rather screwed as there is no getting it apart at this time. It's no fun potentially destroying 35+ feet of aluminum toe rail to re-bed it. Sadly they don't even begin to make Anti-Bond in large enough cans to begin tackling this job...

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post #7 of 10 Old 11-10-2009 Thread Starter
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mainsail,

well, this is the point. i am into a rebuild of sorts and changing the method of attachment of the bulkheads (from a few bolts to tabbed). so the deck can be lifted as high as i want now, as the bulkheads are clear, but won't be able to in the future.

i need to get ahold of the tech support at plexus, but no one answered today. open time is going to be an issue for sure. i picked this product becaue it has the longest open time i could find on their website. i believe the 400 series is more commonly used for this application.

redoing bolt by bolt is not going to work as the issue is not so much leakage at the bolts (more typical i think), but larger quanties of water (green) coming through when heeled over in bigger seas. this i think is because there is no "cover" for the joint, and the water is forced in by the natural action of this activity we love to do (subject our boats to??).

i don't feel like doing a complete fiberglass job, but with the plexus one might be able to get away with a more minimal fiberglassing project.

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You will have to place the sealant/adhesive on and bolt down the entire deck pretty fast in my opinion based on their specs. I think I'd use something with more working time.
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sense it is a 2 part system you may be able to extend the pot life by keeping both the parts in a cooler with ice before mixing, or even the mixed stuff in the cooler. when done with epoxy it wont cure even over night ( slow hardener ) until it warms back up when out of the cooler
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Of course, chilling the components may make them more difficult to mix properly—as viscosity increases as temperature decreases generally.
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sense it is a 2 part system you may be able to extend the pot life by keeping both the parts in a cooler with ice before mixing, or even the mixed stuff in the cooler. when done with epoxy it wont cure even over night ( slow hardener ) until it warms back up when out of the cooler

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