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post #11 of 55 Old 06-21-2008
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A few things I found off the web:

The fiberglass cloth used in fiberglassing a typical plywood boat must be compatible with epoxy resin. Although most cloths will work with epoxy, there are some that are not compatible. If in doubt, test the material using scrap wood, to insure that it cannot be pulled off. Fiberglass mat should not be substituted for cloth.

Fiberglass Mat also known as Chopped Strand Mat or CSM
Fiberglass Mat is a non-woven material typically used for laminate build-up and repair work.

Fiberglass Mat consists of glass fibers laid randomly across each other and held together by a binder.

Polyester is the preferred resin for wetting out this form of glass fiber material.

S/V Scheherazade
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I had a dream, I was sailing, I was happy, I was even smiling. Then I looked down and saw that I was on a multi-hull and woke up suddenly in a cold sweat.
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post #12 of 55 Old 06-21-2008
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Correct. My new nesting dinghy arrived on the roof of an SUV without incident last week thanks to seamanlike lashing. Upon releasing said lashing, a 30 knot gust blew the 55 lb. aft part off the roof (denting it) and onto the concrete (denting the boat and damaging a child's palm-sized piece of gel coat).

Dammit.

Anyway, I used West System 105 packets (about 15 g. each) thickened with collodial silica to patch the mess, and the dinghy buider sent me a wee bottle of matching gelcoat. I'm checking today to see if it set up properly.

When I use straight epoxy, the ratio is usually 2 to 1, but straight epoxy isn't the correct choice for mat. Wetting out tape or cloth for tabbing, drill and fill, or for encapsulating marine ply, yes.

Read Don Casey's Hull and Deck Repair. All you need to know in about 60 pages.
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post #13 of 55 Old 06-21-2008
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Free-

No, you're right. Some CSM mat is setup with a sizing that is incompatible with epoxy resins. The sizing is designed to dissolve in polyester/vinylester resins. Some CSM mat is designed for epoxy resin, but that generally isn't the case.
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Please correct me if I am wrong, but I have read that you should not use epoxy when using fiberglass Mat.

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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
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post #14 of 55 Old 06-21-2008
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Epoxy works fine with mat. Mixing and matching polyester and epoxy is what gets you in trouble. people use polyester with mat a lot because they can. Polyester is cheaper. So if you are casting a bunch of fiberglass parts, you use mat or chopped strand and ployester.

Interesting - seems others disagreed while I was posting. I am not sure I see how it is incompatible. It can be harder to wet out properly. Further edit - the few times I have used mat were for pretty sizable problems on a fiberglass runabout. I soaked it good before slopping it on/in and it dried to a nice solid mass.

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post #15 of 55 Old 06-21-2008
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BTW, even though it appears I might be an idiot also, one other thing for the OP - do not put a coat of properly mixed epoxy down before you talk to the vendor. They may tell you to (I sort of doubt it), but I think that would seal in the goop that might cure slowly if you don't seal it.

-Andy
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post #16 of 55 Old 06-21-2008
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OK, I looked it up and I see the issue. I am an idiot also. When I "repaired" that runabout, I filled in some problem areas with mat that was soaked with epoxy and it dried hard. But all I did was epoxy fill the void with a wet rag in there for good measure. The epoxy did not dissolve the glue and the glass fibers did not bond properly. Good thing for me they got rid of negatives...

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post #17 of 55 Old 06-21-2008
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That doesn't seem to be the consensus from most of the manufacturers, at least in from what I've seen. From one website that sells composite materials and supplies.

Quote:
Chopped strand mat contains a binder which prevents proper bonding with any epoxy resins. Use our polyester or vinyl ester resins with chopped strand mat.
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Epoxy works fine with mat. Mixing and matching polyester and epoxy is what gets you in trouble. people use polyester with mat a lot because they can. Polyester is cheaper. So if you are casting a bunch of fiberglass parts, you use mat or chopped strand and ployester.

Interesting - seems others disagreed while I was posting. I am not sure I see how it is incompatible. It can be harder to wet out properly. Further edit - the few times I have used mat were for pretty sizable problems on a fiberglass runabout. I soaked it good before slopping it on/in and it dried to a nice solid mass.

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—Cpt. Mal Reynolds, Serenity (edited)

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Chopped strand mat, in fabric form, is sold on the roll and in small folded packages. It is made up of 1"-2" long fiberglass strands that are randomly oriented and typically held together with a styrene-soluble binder that acts like glue connecting the fibers. The binder is designed to dissolve upon contact with styrene in polyester resin or vinylester resin. Once dissolved, the fabric softens, allowing it to drape around curved shapes Epoxy does not dissolve the binder.

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post #19 of 55 Old 06-21-2008 Thread Starter
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That book looks good. Thanks.

Click here for preview.

The site i bought the epoxy from also sold me the csm.
Its 300gm sq m

Ive used it twice and it looks strong as steel.

My main experience of fibreglassing up until a couple of weeks ago was watching my dad fit out a 19 ft twin keeled Caprice.
He used Polyester and CSM that matches what i bought (as far as i can remember).

Ive been informed Polyester is not the best choice even if its cheaper

"polyester resins will not give a good bond to wood, in fact it does not really even stick at all, when polyester cures it shrinks by around 6% and it is that shrinkage that people think is a bond. Once any moisture gets behind the resin it will release and your joint at best will come loose at worst it will fail.
I know I am a producer of epoxies but I have also been building boats for 47 years and I went through the polyester phase when we where told how wonderful it was for boat building if you will take my advice use epoxy for all structural bonding"

This is the thread in the Song of the Paddle Kayaking Forum.


Ive also seen it used all over the place on You Tube etc.

Are you saying some grades of matting are designed for Polyester and some for Epoxy?
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IIRC, the do make a few epoxy-compatible CSM mats, but they're much harder to find and nowhere near as common, so avoiding the use of epoxy and CSM is probably a good idea in general.
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Chopped strand mat, in fabric form, is sold on the roll and in small folded packages. It is made up of 1"-2" long fiberglass strands that are randomly oriented and typically held together with a styrene-soluble binder that acts like glue connecting the fibers. The binder is designed to dissolve upon contact with styrene in polyester resin or vinylester resin. Once dissolved, the fabric softens, allowing it to drape around curved shapes Epoxy does not dissolve the binder.

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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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