Plug and Mold for 18' sailboat

Discussion in 'Fiberglass and Composite Boat Building' started by John Merritt, Oct 1, 2018.

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  1. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Thank you ondarvr.

    One of the things I like about this forum is that there are some truly knowledge members who gently correct when a modest member gives poor advice

    John, ondarvr is one who I seek out when I have a resin or glass question. Heed his words..
     
  2. messabout
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    messabout Senior Member

    At the risk of being a party- pooper, I have not yet seen the design that the OP is planning to build. Is it an adequate, carefully and knowledge based design or is it the product of a stylist or predetermined notions. I certainly mean no discredit to the OP but why not post some basic drawings of the proposed build. There are experienced contributors here who might very well be of service in terms of design critique.

    The time and expense of building a plug and mold and then the boat, are surely worth some conversation about the fate of the end result.
     
  3. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    Ondarvr... thank you for the info on continuous filament mat. Messabout, I haven't posted the drawings because I don't know how.... I'm definitely not the most computer savvy person. Ondarvr, if I may, another question: In your opinion what is the lightest weight CSM that can be used between two layers of heavy WR.... and what weights of CSM would be suitable to use as a veil ?
     
  4. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    Wow... been a while since my last post. Work continues apace... just building the strongback was an exercise in engineering. But it is done and all the stations and the stem are set up and ready for the longitudinal wooden strips over which I will afix the foam "planks". My intention is to then paint on eight coats of barrier on the foam ( alternating latex paint and thinned wood glue). This should enable me to use regular resin and csm (two light layers) to tie everything
    together before applying the "bondo coat". ( looking for feedback on all this BTW). My real question has to do with the "bondo" coat that will be sanded to a smooth finish. Automotive bondo seems like it would be very difficult to work with as it is so hard/dense... difficult to sand.. ( and now that it is set up I can visualize just how large the surface area is) Is there a better product that is easier to sand yet will still give a good finish.... and doesn't cost a fortune. I would also like to know any tips for applying a layer of even thickness. Thanx again.
     
  5. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    What is "regular" resin?

    Adding micro-balloons to resin makes a great fairing compound.

    It is often recommended that first time fairest apply the first coat of compound with a notched trowel. When it hardens, fill in the grooves with a second coat and a smooth trowel. Then let the long boarding fun begin.
     
  6. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    I meant polyester resin (not epoxy). I would like to trowel on a total of 1/4" of fairing compound to fair the plug. I had envisioned a first coat made of laminating resin mixed with microballoons etc. spread with a 1/4" V-notched plastic adhesive spreader held at 45 degrees followed by a second layer made the same way but spread with a flat spreader ( all in an attemp to get a fairly uniform thickness) ... this first layer could be colored with a bit of the blue chalk powder used in a chalk line and available at most hardware stores. After a bit of sanding to knock off the high spots I would repeat the process again but with red or orange chalk added. The last coat ( red chalk, smooth spreader) I would add the "wax" to the laminating resin so that it would develop a hard ( non-gummy) surface for the longboard sanding. The two different colored layers would give me a visual clue if I was removing too much filler in any one area.

    So ( being a total Newbie) I'm looking for (hoping for) a "recipe" for the different additives to add to the resin ( microballoons, fumed silica, cab-o-sil, wood flour...etc) to make this magic sandable fairing compond... plus any and all tips about where to get materials at a decent price, how to mix... etc. I plan on using glass microballoons due to cost and the sides of the plug are vertical.... so may need more thixotropic? agent for these surfaces. Thank you.
     
  7. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Why go to the trouble of using foam at all? Just strip plank the mould. The wood does not have to be high quality, whatever is cheapest is fine, even reclaimed wood. No glue necessary between the strips, just screws to the moulds. Then you sand it smooth and reasonably fair and apply a layer of CSM (whatever weight is cheapest) with polyester.
    Now you can apply your favorite fairing compound and longboard it to perfection. Automotive filler (bondo) is cheap and made to be sanded, no need to use something else.
    You can actually fair most of the mould in wood. If you use thicker strips and close mold spacing you could even skip the csm layer, just use bondo directly over the wood.
     
  8. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    I have thought about that. However, if I wanted to mix my own fairing compound I would like to hear a few "tried and true" recipes from the experts. Thank you.
     
  9. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Bondo will never give you a proper finish for a mold. It has a rather grainy and porous surface. Also, it is very soft which is the opposite of what you want on a mold. Molds are built much thicker and heavier than the object that will be laminated in them. Further, they have reinforcements laminated on them (steel pipes, etc.) to keep the molds from warping and distorting.
    The most obvious drawback is that you are making a mold for a design that may be terrible. The usual method is to build a prototype, test it, modify until it meets expectations, and then use it to make the mold.
     
  10. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    If you use cheap EPS foam you can use StyoroShield over it and then use any resin you want.

    Making a fairing compound isn’t difficult, but there is no “one size fits all” recipe.

    The exact resin you plan to use will have a huge influence on the ratio of each ingredient you plan to use. Plus you may want to adjust ingredients and ratios depending on exactly what you plan to do with that batch.

    The size and shape of the plug will determine how strong you need to build it. Plugs are frequently destroyed when you tried get the mold off it, so if you plan to re-use the plug it will need to be built stronger.

    Molds are normally just made with CSM, if any other fabrics are used they’re buried deep in the laminate, or behind a core.

    There are special tooling resins, they’re not required, but do build a better mold.
     
  11. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    Thanx all. The fairing compound is just for the plug, not the mold itself. While I understand that every different resin may require a different recipe to make a good fairing compound, I need a place to start. Can you recommend a specific resin that has worked well for you in the past and a recipe of the additives that you have used with that resin for a similar application.
     
  12. gonzo
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    gonzo Senior Member

    The laminate schedule you propose is too thin. If you are strip planking, there is no need to add foam on top. Use 3/4" pine and that will give you enough material to plane it down. Make sure you set in the screws holding them to the stations.
     
  13. John Merritt
    Joined: Oct 2018
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    Location: Ada, MI

    John Merritt Junior Member

    I am not strip planking. I am using foam. Lots of folks here must have experience mixing up their own fairing compound. I am just looking for advice on how to mix up my own. Anyone ?
     
  14. ondarvr
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    ondarvr Senior Member

    Most people don’t have a set recipe, you mix as needed.

    Silica stops it from sagging, talc can make it smooth and sand easier, micro spheres can make it very easy to sand, milled fibers can add a little strength.

    Make a small batch with the resin you buy, you’ll get it dialed in for your liking with that exact resin quickly.

    Sorry, that’s the best I can do.
     

  15. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    My technique for fairing compound:
    Make peanut butter with micro-balloons. Add a scoop of silica. Two scoops if vertical. Three if overhead.
     
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