Confused on the order to do layup

Discussion in 'Fiberglass and Composite Boat Building' started by Buckeye492, Aug 12, 2019.

  1. fallguy
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    fallguy Senior Member

    Okay, but what about the boat repair yard that suggests polyester is stronger than epoxy? That ought to bother a few people. Even when you defend their wisdom with, yeah, well, it works nice for gelcoat..
     
  2. ondarvr
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    ondarvr Senior Member

    I haven’t heard the “it’s stronger than epoxy” opinion, you can’t really defend that. All you can say is that for most jobs it’s good enough.
     
  3. Deering
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    Deering Senior Member

    I gotta agree. You nailed it. I may even have been that guy once. It applies equally to a thousand other projects, be they painting, woodworking, ‘gourmet’ cooking...
     
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  4. fallguy
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    fallguy Senior Member

    Oh, I got into it with a woman boatyard operator online...
     
  5. Buckeye492
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    Buckeye492 Junior Member

    Ondarvr, do you see a problem with using just 1708? Is it overkill for what I am doing? I am open to suggestions.
     
  6. ondarvr
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    ondarvr Senior Member

    Nothing wrong with it, it’s a good product, it’s significantly better than what was used in the original build, and you’re using more of it, which is fine.
     
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  7. Buckeye492
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    Buckeye492 Junior Member

    How many overall layers would you say is enough? I figure I will have 5-6 layers in the cracks the 1 or 2 over the entire surface. Will that suffice? I only ask because I am concerned about the overall thickness & any problems it may create when mounting the motor.
     
  8. fallguy
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    fallguy Senior Member

    Well, personally, I would use epoxy and avoid 1708 and use 17 oz biax. The biax can be snaky, so you have to use rollers and reference lines and prep/cut it dry; no small task. The upside to 1708 is it will hold a shape, but I still hate wetting it out. You basically fill or coat the transom with thickened epoxy; let it gel; roll epoxy over the thing and start rolling layers out of the sun with a 60 minute cure time and never above 80F. Then you can just keep going all day until the layers are done; they will warm, but not burn.

    The reasons for epoxy are that your bond area glass to hull is really short and I would prefer all the possible strength I could get.

    And 1708 is a pita to wetout.

    If you use 5-6 layers of 1708; you are going to develop one heluva hard spot on the corner of the transom. 1708 runs about 0.040" per layer or 6 layers is just under 1/4" thick. This is why I hate the exterior approach. Done right you'd radius and wrap the hull sides and bottom, fair and repaint. Lotsa work versus the inside.

    Ondarvr won't steer you wrong, though.

    Maybe you build with a hardspot and grind it back or just make all of it with 1708 and shorten each layer 1/2" or so. The count of layers seems a bit high, but people get sued if you build it poorly with 4 layers and they blame the guy who told you four layers and wrap the hull when you built 4 layers, no hull wrap and used a weaker glue to save 100 bux.

    My project, I use epoxy, 5 layers of 1700 tilted each time, but only if they fit and wrap the hull 10",8",6",4",2" side and bottom. I would also laminate the coosa with vac or at least weights to 1.5" on the table prior to embedding into the existing skin. If you use vac, 10" of mercury works real well with a peanut butter thixo applied to both sides woth a 1/16" u trowel. Pin it, though, it will like to move on you under pressure. If you use weights; put some 2x8s across the boards before the weights and still pin it. We use aluminum nails to pin stiff; they sand easy..

    Correction: how are you going to ise 5-6 layers in the cracks? You use a thixotropic mix in the cracks. You have to grind out all the gelcoat on the sides. Submit a new photo of the work.
     
  9. fallguy
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    fallguy Senior Member

    boat-coosa-glass.jpg

    Here is a quick finger sketch.

    Black is the boat.

    Blue is the coosa.

    Green is the glass.

    All white areas must be filled with thixo. Not too big an area or it will heat and crack.

    The glass must extend well past the cracks. I prefer a hull wrap, but you have a pretty big area on the sides and bottom to extend the glass. If the transom is not all planar; let us know. You don't glass over gelcoat, so it must be removed from the transom edges for this repair.
     
  10. fallguy
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    fallguy Senior Member

    The thixo mix is typically the resin and catalyst/hardener and cabosil and/or milled glass. Although, I would skip the glass. When you bed the coosa into the existing boat; you use the same stuff and when you bolt the coosa to the hull; it squeezes the excess out and into those grooves; then u pull the bolts prior to cure. Then you force more thixo into the sides with a caulk gun or trowels. Too much will heat or catch fire. Too little and you can't get a good glue joint.

    The thixo mix is made with cabosil and epoxy or your glue of choice. I like to mix it on a board. If it is too loose; it will run right out of the seam. Pile it on the board and watch it. If you see it sag it is too loose.

    The same thixo mix is applied jist prior to glasswork to fill lowspots so you avoid air in the laminating process.

    By volume, a good guide is about 1 epoxy to 2.2 cabosil and then add more cabosil or epoxy as needed. An example is 6 oz epoxy to 13.2 cabosil..by weight would be more accurate..jist haven't bothered..
     
  11. Buckeye492
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    Buckeye492 Junior Member

    For the exterior, my plan was to grind a 12:1 bevel around the crack then lay progressively wider layers of 1708 to build up to the finished thickness. Original thickness was 1/4", so I was estimating 5-6 layers. Once that was complete & cured, prepare the entire surface & lay 1, maybe 2, full sheets of 1708 over the entire surface & around the bottom and side corners. Then fair & paint. A lot of work, yes.
     
  12. fallguy
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    fallguy Senior Member

    Not sure how that will level out, but i understand the plan. I would just 5 layer itall with 1700. You would level it out with thixo.
     
    Last edited: Aug 25, 2019
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  13. ondarvr
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    ondarvr Senior Member

    You can’t wrap around the outside of the transom unless you first grind off the same thickness you plan to add.

    Wrapping onto the bottom without grinding will leave a build up that will cause issues when planing.

    This is one of the downsides of doing the repair from the outside, if you don’t leave a large enough flange for glassing, the bond area gets tight.
     
  14. Buckeye492
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    Buckeye492 Junior Member

    If I wrap around the bottom it stops after 3" where it hits a vertical portion where all the thru-hull fittings are mounted (sorry don't have a picture, but can take one). Around the cutout I left a 3" wide perimeter based on a 1/4" skin thickness. I figured 6 layers of 1708 @ 0.044" per layer puts me about 0.014" over the original thickness. or should I do 5 layers & be slightly below the original?
     

  15. fallguy
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    fallguy Senior Member

    Get us a picture and give us dimensions of the surface. The picture I drew is what is anticipated or expected and it is obviously not fully valid here.

    Ondarvr's point about wrapping the hull surfaces is really important. I suggested wrapping as an ideal, but it is not possible here. And since wrapping is an ideal, replacing transoms from the inside is much better; despite polemics, because very little finishing work is required and you don't mess with planing surfaces which have in theory already been built correctly. You are well past all of that, so now you go for practicality.

    I don't generally like the idea of filling a bunch of short tapes into the 'crack' because the long bond surface of the full glass pieces are really preferred not sandable. Does that make sense to you? In other words, in general terms, you want the long bonds to be untouchable by the sander. This also means you should try to finish low and not proud. It would be better to fill the void with cabosil and lay in the 1-3 long panels of 1708 and then infill the 'cracks' with more pieces of glass and this is what is done by industry professionals. It is counterintuitive, but correct. Consider for a moment you install 6 pieces of filler, then you install a single piece of 1708 and the 1708 is the longest piece. You end up sanding it all off in fairing and then your 1708 is actually supposed to be the strongest bond and ends up the weakest.

    The difficulty here is we don't quite get the picture because it sounds like the core is relieved or short of plumb to the existing ground down transom.
     
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