Fiberglass Cloth or Fiberglass Mat? Fiberglass Resin or Epoxy Resin?

Discussion in 'Fiberglass and Composite Boat Building' started by canoe42, Jun 29, 2008.

  1. canoe42
    Joined: Sep 2007
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    Location: Maine

    canoe42 Junior Member

    Need some help in choosing the right material to repair a fiberglass canoe with one small 1/4" hole and a spider cracked gelcoat covering.

    I know how to repair the hole with fiberglass resin and hardiner. Need to know if I should use fibergalss cloth or fiberglass mat. Have heard the cloth is stronger and best used to build up a repair in layers, then use fiberglass mat as the last and final layer over the repaired area because it is more abrasion resistant then fiberglass cloth.

    Mending gelcoat - I do not plan to remove and replace all the gelcoat. Have been told it is not necessary and satisfactory results can be had by sanding out as much of the cracks as I can with 80 grit sandpaper, fill the remaining cracks with something durable, finish sand and paint. Comments from this and other web sites suggest filling in the small spider cracks that are left with a variety of products such as fiberglass resin, epoxy resin, epoxy spraying putty, white polyester putty or gray autobody putty. When filling deeper cuts and gouges it was suggested to mix colloidal silica into epoxy resin to thicken it. I do not know the difference between these products. Is fiberglass resin with hardiner the same thing as epoxy resin? My guess is it is not, based on the following ad I found,

    System 2000 Epoxy Resin. It is a low viscosity, light amber laminating resin used to maximize the physical properties of glass laminates. Test results have proven superiority over other room temperature epoxies. Its low viscosity and great handling characteristics make it a favorite. 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.

    Have sanded the entire canoe with 80 grit sandpaper. Cracks are visible but very shallow. The 1/4" hole has been sanded down to fiberglass and beveled out 2" around the hole. Hopefully someone here knows what would be the best products to use.
  2. the1much
    Joined: Jul 2007
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    Location: maine

    the1much hippie dreams

    ya got any pics?,,,and you should use "cloth" to fill the hole,,should be an inch or 2 bigger then ya have beveled out already,,,then mat last ,,,,NOT because its more abrasive (it isnt) but because it gives an easier surface to fair.,,, you should use epoxy if you dont know what your canoe is made from,,,,and whoever told you to sand the cracks till whatever then fill,,,is an idiot,,you need to grind the cracks till their gone,,,and you need to (V) grove them all,,,or you'll do all this work and a month later the cracks will be back.
    and you can use micro-balloons or cabosil to thicken epoxy.
  3. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    Unless there's some reason to believe your canoe was built from epoxy, I'd just go with polyester resin. Mat is comprised of short, randomly-oriented fibers. It's not as strong as cloth, but how much strength do you need on a 1/4" patch? If you have cloth available, go ahead and use it, but mat will work as well.

    The success of your patch is going to depend more on your craftsmanship than on the materials. The edges of the hole should taper no less than 8:1 - sounds like you've nailed that. There should be NO surface your patch touches that hasn't been ground down to "virgin" material. If, as you're sanding the patch, you see a seam at edge - no matter how tiny - that's where it's going to start coming apart later.

    Use no more resin than necessary to wet out your glass. Most amateur repairs I've seen were terribly resin-rich. Sop up any excess resin. (I used cheap paint rollers when I did this stuff for a living, but rags will work on small jobs.)

    Spider cracks indicate impact damage. Sand off the gelcoat entirely on one of them. If you see white lines in the fiberglass itself that replicate the gelcoat cracks, another patch is in order, just as if the spiderweb was a large hole. You need to get rid of all the cracks by grinding them out and patching, just like you did the hole. (In fact, you'll make holes.) This is where I'd choose cloth over mat for the repair.

    The easiest way to patch impact damage is to grind one side down to virgin material, then lay a few layers of mat against it. This will help you maintain the shape. Now go to the other side and grind out the cracks, making an 8:1 taper. (If the glass is 1/8" thick, then your taper should be 1".) Patch this side, then go back and grind off all that mat you applied and patch the other side correctly.

    Wearing proper sunglasses makes gelcoat cracks seem to disappear, and in some cases this is preferable to spending the rest of the summer patching more utilitarian items. But I don't know how valuable your canoe will be when you're done.
  4. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    tinhorn has nailed it, do exactly as he says and you will sleep in peace.

  5. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    This is very good advice. Decals can also be used for cosmetic repairs, as well as obfuscation. "Look at that naked woman over there" can hide many imperfections over here.

    With the 1/4" hole, some canoes are only two laminations of 10 oz fabric and very thin. A smaller patch on both sides may be better than a bigger one on one side, I.E., beveled out 1" on both sides. Some canoes are very flexible, so you want the repair to come close to matching the flexibility of what's there so you don't create a hard spot which will crack the gelcoat or the adjacent laminations over time. If it's a heavy, clunky chopped strand canoe, it hardly matters.
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