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Old 20-12-2009, 13:13   #1
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Polyester vs Epoxy Resin on Divinycell

I have built and shaped a sea hood out of divinycell foam. I now need to cover it in several layers of fiberglass. I notice that polyester resin is substantially cheaper than epoxy resin. Is there any reason I canít use the polyester resin for this job?

Also does any one know how to calculate how much resin i will need?

thanks
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Old 20-12-2009, 13:59   #2
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Polyester should be fine for this job. Hard to say how much you need without knowing how big it is. I would think you might use 3-4 quarts depending how big it is and how much you end up wasting. Remember to hot coat the foam prior to putting the glass on.

good luck
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Old 20-12-2009, 18:28   #3
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I hate to sound ignorant but I am, so -
What's a sea hood?
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Old 20-12-2009, 18:34   #4
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Polyester has a hard time sticking to itself. Pay extra - go epoxy. You will thank yourself many times in the future.
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Old 20-12-2009, 18:42   #5
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Quote:
Originally Posted by Roy M View Post
Polyester has a hard time sticking to itself. Pay extra - go epoxy. You will thank yourself many times in the future.
Sorry do not understand what you mean polyester has a hard time sticking to itself?
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Old 20-12-2009, 18:45   #6
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Thanks for all the info. A sea hood is like a pocket that the companionway hatch can slide into. Some boats have them molded into the deck and some boats don't need them at all. With our Bristol 29 we have found that a larger boarding wave crashing into the front of the closed hatch still finds its way into the cabin so that is why we have decided to build this.
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Old 20-12-2009, 18:52   #7
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If you abrade poylester properly, 80 grit, it will stick to itself or anything. If you use an epoxy such as West Systems that amine blush, the blushing property gives a working window, depending on temp, 3-10 hrs or there abouts. System 3 dosn't blush so the working time is much shorter or maybe not even possible without abrading.
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Old 20-12-2009, 18:53   #8
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Hi fullKeel I was thinking you were talk a dodger I will revise that to 1-2 quarts of resin. That will teach me to read better lol. What material are you using 1808? I would think 2 layers of the would be fine maybe add a 3rd if you think you will be walking on it.

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Old 20-12-2009, 19:06   #9
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I would use polyester no reason to use epoxy above the waterline.
Two types of resin are usually available laminating and finishing. You want laminating for tha lay up go figure. Finish it with uhh finishing. We used to add a wax to laminating rain to make it a sandable finish. Laminating resin sands horribly.
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Old 20-12-2009, 20:34   #10
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There is absolutly no good reason to use epoxy for this job, since you are not using a female mold you are going to have a lot of hand fairing and finishing to do and the job will go much faster with polyester resins and fillers,tens of thousands of seahoods have been made quite successfully with polyester without the world coming to an end as the one size fits all use epoxy for everything folks would like you to think.
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Old 20-12-2009, 20:43   #11
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You will use approximatly the same weight of resin as glass and this you can calculate based on the square footage of the project and the laminate you intend to use.
Steve.
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Old 21-12-2009, 09:03   #12
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Drop a winch handle on a polyester/fiberglass plywood deck. Do the same on an epoxy/fiberglass deck. Wait three months. Observe. Polyester yields delamination, a bubble which then cracks and rots the substrate. Epoxy yields a "star fracture" at the point of impact. Less damage, no delamination. Polyester will also delaminate from itself if conditions were not perfect at application. Epoxy is much more forgiving and stronger. But go ahead and choose what makes your bells ring. If it's on your boat, you get to accept the consequences. If it's someone else's boat, it's now someone else's problem. Too bad.
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Old 21-12-2009, 09:20   #13
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Quote:
Originally Posted by Roy M
Drop a winch handle on a polyester/fiberglass plywood deck... Polyester yields delamination...
Actually it was a 22ozs. hammer, not a winch handle, but I've done it and it did not delaminate. Methinks you've had a bad experience with some poorly laid-up polyester. Done properly, for this kind of application, polyester is every bit as good as epoxy.
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Old 21-12-2009, 09:29   #14
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Roy every material has its place and use. If I were putting a thin layer of glass over a plywood deck I would use epoxy too. In this case he is glassing over a foam core on a deck part. There is simply no reason to go with the expense and trouble of epoxy. If he does use epoxy he has to use special Glass with the proper binders increasing the cost even more. Epoxy is not as forgiving to work with as far as mixing and temps.No point in increasing the difficulty factor for this project.

That is what the art of boat building and repair is all about finding the right combination of materials to work best for a given application. There is no one right product for all uses. Most have advantages and disadvantages each has to be considered for its own merits from the use. Also you have to consider the boat and use. Honer the medium a friend of mine used to say. Why build an expensive hi tech sea hood and put it on a mid range production sailboat. (no insult meant here at all) Everything in life is a balance and this applies equally to boat repair.

Good Luck
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Old 21-12-2009, 10:01   #15
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Roy,you are absolutly correct,no one in their right mind would use polyester resin to glass sheath a plywood sea hood,but thats not what the OP is doing,he stated he was using foam and for this application there is no reason to use epoxy.I see you own a searunner tri,great boats that definatly should be epoxy all the way but sorry,no relationship to what the OP is doing.Polyester does btw stick adequatly to plywood,there are tens of thousands of stick built,production glass boats sailing the oceans of the world with their plywood bulkheads structurally tabbed in with mat/rovings/polyester resin and they are not all falling apart even though this is an obvious place where an epoxy/biax laminate would be far superior.All the different materials have their place,polyester has not been,nor should be made obsolete by epoxy.I know that the Gougeon book will tell you to use epoxy for everything and the more of their product you can consume the better(for them) any boatbuilding pro would know that epoxy is not always the appropriate choice.
BTW,i am just finishing up repairs to a searunner 25 that dragged its mooring,the boat is 28yrs old and apart from the damage has virtually no issues,it was beutifully built with epoxy products,the only glass is at the chines,everything else is just sealed and PU painted and its held up as well as any glass boat of similar vintage,its a sweet little boat.
Steve.
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