finishing a dinghy plug

Discussion in 'Boatbuilding' started by maudlawes, Apr 29, 2014.

  1. maudlawes
    Joined: Apr 2014
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    maudlawes New Member

    have made a plywood plug for a dinghy,can i paint it with normal gloss paint,then rub down a few times,before i polish it,await your expert advice
     
  2. SukiSolo
    Joined: Dec 2012
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    Location: Hampshire UK

    SukiSolo Senior Member

    Welcome to the Forum

    There must be loads of information on this out there. Personally I would epoxy sheath the plug and 2 pack paint it in gloss black to find any imperfections. That is what I have done with any plugs (bucks) I have made and it works fine for me. Cut with 1200 or 1500 and buff to high gloss, the dark colour allows the most minor of imperfections to be shown up.

    After that you need to choose a release system, probably wax (not beeswax) and coat the thing maybe plus PVA depending on eventual construction of vessel, prior to making the mould. Do not forget to allow a little hardening time for the epoxy and paint, like a couple of weeks prior to taking a mould off. At least, I prefer to let it reach full hardness.

    Good luck with it.
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Epoxy can lead to problems. The amine wax in it will prevent poyester and vinylester resins from curing. Use the proper mold gelcoat. It is specially formulated for that use.
     
  4. mastcolin
    Joined: Jun 2005
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    mastcolin Senior Member

    loads of uk builders use durabuild. You can use a good 2 pack polyurethane. In Holland deijssel is used but you can use International Perfection. Speak with these people.
     
  5. Ben G
    Joined: Jun 2013
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    Ben G Junior Member

    I have epoxy sheathed a polystyrene plug with thin woven 'glass. Laminate carefully to make sure the same number of plies so as not to get high spots - plan overlaps to be off the hull surface.
    Screeded and initial fair with epoxy bog. At this point think hard about whether your lovely wooden plug is seaworthy, thereby avoiding further work ;)
    Assuming you still want a mould, spray with high build. I used 'Duratec' Polyester (not epoxy) high build which is designed specifically for this. sticks to epoxy substrate, very easy to sand, polishes up and seals beautifully. Nothing else like it but expensive.
    polish, wax, tooling gelcoat, finish glass, substrate glass, no air bubbles, mould framing, backbone, wheels... then start making your boat!

    Scroll down a bit to see the process I went through:
    http://z10.invisionfree.com/12ft_skiffs/index.php?showtopic=504&st=0

    Cheers
     
  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Nice work Ben G. Good Blog and documentation with some interesting points. I'm surprised the CNC did not blow/suck the foam away a bit more. Having known a couple catch fire before now from build up of removed stock.

    There is a trick with getting PVA (Blue) to sit on carnauba wax. You need to very fine mist spray it. The surface will have a very light texture like 800 grit but it will sit on it. Obviously the mould has this too but can be polished out. Brushing etc is a waste of time as you discovered..;)
     
  7. Ben G
    Joined: Jun 2013
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    Ben G Junior Member

    Thanks!

    The CNC did need occasional clearing of the foam away from the shape.

    I later found out that part of the problem with the PVA over wax was that the wax was a high temp version, which apparently contained a small percentage of silicone. I haven't had the same problem since changing to a carnauba-only wax since then. It seems good - quality carnuaba can be harder to find than I first thought. Would have been handy to know straight up!
     
  8. Ben G
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    Ben G Junior Member

    PS that reminds me - I remember being advised to let various stages stand for a few days here and there. IIRC the duratec should be sanded in the following days after spraying, then left for a few days to outgass and harden. The outgassing solvents can affect the release system??!!
    Also when building the mould, make sure you know the schedule ie application times and dwell times for gelcoating / laminating etc . That way everything just works out.
     
  9. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Thanks Ben. The reason for leaving for a few days is curing cycle. When I have used 2k polyester hibuild undercoat/primers, usually you can cut them back wet in about three hours. However thay are still hardening and will do so for at least another 7 days. Even 2k automotive/marine paints need around 14 days to get full hardness in just air. Baking just accerates this part, and allows the remaining volatiles (carrier) out and the solids to cure.

    Same with resins, polyester or epoxy. So it is a balancing act of getting the cutting window to allow fast sanding and polishing and then letting it cure. It helps minimise stick out.

    Nice looking design btw, be interested in how she does in the heat of battle this year. Please keep us updated.
     
  10. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Do a test first, but normally with polyester for the mold, if you can seal the plywood surface and are able to apply 3-5 coats of wax and PVA, plywood and gloss paint will work. I've used 1 part automotive enamel paint, I've used cheap spray can paint.

    Avoid any materials that will be affected by styrene fumes, like Styrofoam, and let the paint dry/cure good.

    Epoxy is more tenacious so more precaution is needed when coating the plug, but with polyester, stabilized plywood, single part gloss paint, wax and pva works.
     

  11. maudlawes
    Joined: Apr 2014
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    Location: teignmouth devon

    maudlawes New Member

    many thanks for all your information,will see how it goes
     
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