Fiberglass Resin: Polyester vs. Epoxy?

Fiberglassing over plywood and one-off fiberglass methods. See: "Boatbuilding Methods", in the left-hand column of the Home page.

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SammyRye
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Fiberglass Resin: Polyester vs. Epoxy?

Postby SammyRye » Thu May 25, 2006 5:13 am

:D Hey, all -

Here's a question for discussion - while looking at what is available at some of the local stores for fiberglass tape, cloth and resin, I've noticed that there are a few brands available for marine use, but they all are polyester based. Even looking at marine resins available on the net, I've found several boat builder brands in polyester.

Now, there is a significant cost difference between polyester and epoxy, polyester being much less. I've read the evils of not being able to use epoxy over polyester, with polyester over epoxy being okay.

For my latest project, I'm building up a stitch-n-glue 10' pram to be powered by a 40 lb. thrust trolling motor. The exterior and interior of the hull is to be painted with marine paint, with the trim pieces, seats and decking to be left clear. This boat is going to be used for cruising small lakes and quiet rivers and streams, and maybe a fishin' trip or two. I'm not building it to show and don't plan on it lasting for 50 years - what problems or shortcomings am I going to run into by using the polyester resin? Maybe this has been covered here before, but I didn't see any thing current.

Comments welcome - personal use stories (the good, bad and ugly) appreciated!

Thanx,
Rich
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Graham Knight
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Postby Graham Knight » Thu May 25, 2006 5:30 am

Epoxy is much stronger, bonds to the wood better, and is far easier and more pleasant to use.Having said that if you're trying to cut costs, and the boat isn't expected to last very long, you may find polyester is adequate.
We've made several stitch & tape boats at work as film props, in fact we're making a couple of pedalos right now, and we use polyester resin and glass tape because it's cheap and it works just fine.
But they only have to last the duration of the shoot, I know some have been taken home by crew members afterwards but I couldn't tell you how long they've lasted.
They were all tough boats though, they have to be because props guys are notoriously heavy handed and will break anything they can!

One thing I would mention, consider the cost saving on a 10' boat, you won't be using very much resin if it's only for taping the seams, so the saving will not be as great as if you were using gallons of the stuff.
Graham in Shepperton, England

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ET
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Postby ET » Thu May 25, 2006 9:56 am

Remember that boat builders build in wood/epoxy as well as fiberglass. Polyester resin sold as a "boat builder material" is probably intended as laminating resin for those either building a fiberglass boat from scratch or for repairing a fiberglass boat. As mentioned above, using Epoxy for your wood boat project is definitely a best practice, and as also pointed out, the cost savings on a small boat would be negligable.

basilkies
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Postby basilkies » Wed May 31, 2006 3:01 pm

You want marine epoxy, no discussion, it is more durable and stronger. Get Glen-L's stuff, West System, Tap Plastics or look at a Jamestown Distributers Catalogue. It sounds to me like you are looking at the local hardware store.

terrymc
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Postby terrymc » Sun Jun 04, 2006 8:17 am

Amen to all of the above, but let me emphasize one other point...polyester STINKS (as in odor, not performance) and epoxy does not. If you use polyester, the smell will permeate the entire building where it is being used. I used polyester on my first boat (30+ years ago) and my wife still complains about the smell! Use the epoxy.

Peace,

Terry McIntyre
MOrgan Hill, CA

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BORD
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Postby BORD » Sun Jun 04, 2006 9:38 am

I've read the evils of not being able to use epoxy over polyester, with polyester over epoxy being okay.


Correct me if I'm wrong but isn't this backwards? I thought you could use epoxy over polyester but not polyester over epoxy.

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Graham Knight
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Postby Graham Knight » Sun Jun 04, 2006 1:09 pm

You're right, it's wrong.
Graham in Shepperton, England



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SammyRye
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Postby SammyRye » Sun Jun 04, 2006 6:13 pm

Okay, it seems as though somewhere along the way, someone has convinced boatbuilders that ALL wooden boats should use epoxy versus polyester resins.

In Fiberglass Boatbuilding for Amateurs, Ken Hankinson talks extensively about the pros and cons of epoxy and polyseter for fiberglass boatbuilding. He leans towards polyester, and explains his reasoning. This can be found in Chapter 6 of his book.

In Boat Building with Plywood, both Glen L. Witt and Ken Hankinson write of using polyester resin for fiberglassing. This can be found in Chapter 23, Finishes & Fiberglassing, pages 220-226.

Like I said, they talk of the pluses and minuses of both resins, but prefer the polyester over the epoxy for several reasons, and one of those is cost. Polyester resin with hardner for marine application can be purchased for $22.00 / gallon. The least expensive epoxy resin I found was $57.00 / gallon - not including the hardner.

My little pram calls for 3 gallons of resin. Now, for my use, expected life of the boat and cost of other building materials, I'll go with the $66.00 in polyester rather than the $171 + hardner ($200?). I can stick the savings into something else.

Don't get me wrong, I can fully appreciate the pluses of epoxy, but I can also understand the economics of boating. Polyester is fine for my $200 boat, but epoxy goes onto my $2,000 boat that'll be passed on to my grandkids.

And, yes - I'm shopping at my "local" building center - maybe that's where Glen and Ken shopped?

To each thier own - I just thought I'd pass this along to anyone interested - good book, too.
Rich
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Graham Knight
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Postby Graham Knight » Mon Jun 05, 2006 5:49 am

I think that's more or less what I said in answer to your original question isn't it?
Epoxy is much stronger, bonds to the wood better, and is far easier and more pleasant to use. Having said that if you're trying to cut costs, and the boat isn't expected to last very long, you may find polyester is adequate.
Graham in Shepperton, England



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