small voids under polyester resin and FG mat

Discussion in 'Fiberglass and Composite Boat Building' started by Brian Peterson, Oct 20, 2012.

  1. Brian Peterson
    Joined: Oct 2011
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    Location: Westbrook, Maine

    Brian Peterson Junior Member

    In tabbing a 2x4 wide side down to the hull, after epoxying the wood to the hull and back wall (no epoxy on top or outside vertical of 2x4) I put down a layer of mat, 24 oz roving, and another of mat to tab it in. It was all good when I left, but upon return after hardening, the top 3/4" of the vertical edge of the 2x4 had its mat/roving pulled off about 1/16th inch, about 18" long..
    This install is reinforcing a transom along the hull in a 17 foot runabout. The tabbing to the transom is good, as is the top of the 2x4.
    Is this something that should be all ground out? Can I just inject poly in there to fill the void? Or not bother at all?
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Okay, some basics, are you using epoxy or polyester? As you've mentioned both and typically poly over epoxy is a lousy bond, but not the other way around. Second, it's not recommend to epoxy bond wood thicker than 1" ( hardwood, you're best with 3/4" laminations), as the internal stresses in the wood can overcome the bond line. Third, you're using mat and roving, which aren't especially useful with epoxy laminates on wood. Lastly, was the wood encapsulated (epoxy or polyester) prior to bonding?
     
  3. Brian Peterson
    Joined: Oct 2011
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    Location: Westbrook, Maine

    Brian Peterson Junior Member

    Hi PAR

    I'm the guy who has had some contact with you last Fall - am rehabbing the 17-foot 1958 Seabreeze Contessa. The transom was one quarter gone - had a big hole in the SB waterline, which extended under the transom chine over halfway across the bottom. This went under about 1" so it wasn't too far towards the bow, but it was a problem.
    I since have added a transom made of plywood (exterior) 1.5" thick, made of two 3/4" plys, epoxy and cloth between, cut to fit within 1/8" of the profile all around, coated 3X with poly, deck puttied in, with mat, roving, mat, roving, mat 4" up and under the hull inside and out , and along the motor tray top and inside bottom.
    In talking to some boat repair people here in Portland, Maine, they suggested putting 2x4 "bracing" along the sheer and hull/transom bottom, to add strength to the part where it went under the transom and up the SB side. I did the whole transom both sides.
    The 2x4s were profiled to the hull and sheer. I understand the poly/epoxy problem, so epoxied JUST the bottom and sides touching the hull. The top and outside sides of the 2x4 were wet out with poly for the tabbing of mat and roving.
    All of this seems very solid, but it's that 3/4 x 1/16th " space along the 2x4 side top I don't like ....am just uncertain .
    I'm thinking that if I grind out JUST the bubbled area, that will kill the mat/roving "bond", strength, what-have-you, so in that case I should just take the whole thing out (which I would REALLY not like to do...) . On the other hand, if JUST that area is the problem, then the epoxy to the hull, and the tabbing on the top and sides will most likely hold the whole thing together. Injection of poly into that space would fill it up, but I'm thinking the wax would negate any bonding, so I'm just kidding myself about the strength buildup?
    I will be putting a 50HP Mercury on this. I tend to get into overkill when I do things, but am uncertain if I am kidding myself about the strength of this whole thing.
    I guess the question is "How much strength does a small void subtract from the whole?" Or is this not a "small void"?
    Right now I am planning on tabbing the stringers up against the transom to add to the whole system. They will be tabbed to the 2x4s to create the drainwell in the middle bottom.
     

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  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You have used polyester resin over epoxied surfaces?
     
  5. Brian Peterson
    Joined: Oct 2011
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    Location: Westbrook, Maine

    Brian Peterson Junior Member

    No

    No- look at the picture of the solo 2x4 -the epoxy (thickened) was applied to the space between the transom and the vertical side of the 2x4, and the 2x4 horizontal side on the hull. After weighting, Any excess was wiped up, and the contiguous transom / hull surface acetoned. After sanding the transom and hull next to the 2x4 , the poly was wet out only on the top and inside vertical surface of the 2x4, then the mat, roving, mat was applied as tabbing for the block. The epoxy was allowed to dry for a week before the 2x4 was sanded carefully to eliminate any epoxy on the top or inside vertical surface.
    I do understand that you cannot put poly over epoxy. I wanted the epoxy in there to really glue it down. Was going to use Gflex, but that is not in the budget.
    In short, evrything was carefully sanded. The hull and transom side of the block got epoxy, the top and other side fo the block got poly and glass.
     
  6. tunnels

    tunnels Previous Member

    There a second problem here as well !! first epoxy is the odd ball in the resin list Polyester and vinylester are pretty compatable and one can be used over the top of the other !!
    BUT
    if you start using epoxy you must carry on with it !!
    Epoxy over polyester or vinylesters is ok but not the other way round round .
    The other issues is did you cove any of the corners ??
    if not then that another reason for the glass to pull away !!!
    Polyester resin shrinks when it hardens so if you try to make a dead right angle corner then it wants to close up as it hardens and shrinks so to speak !,
    If you cove the corner it help reduce that tendancy .
    A plastic water pipe about 20mm in dia is ideal size use polyester bog and the wood i would coat with polyester resin to seal and when its hard then glass over it !! remember the higher the percentage of catalyst you use the more shrinkage you going to get !!!:eek:
     

  7. pescaloco
    Joined: Feb 2006
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    Location: so. california

    pescaloco Senior Member

    Brian

    The most likely reason you have a air void is lack of soft radius edge on your wood. any hard edges will cause the glass to pull off the corner and cause a void.

    Your point is understood (you epoxied only one edge of the wood)
    but yes polyester doesn't bond well to epoxy so if you were not super clean with you application you may compromise the polyester bond to (the clean wood side)
     
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