Resin type choice BEFORE fairing and sanding

Discussion in 'Fiberglass and Composite Boat Building' started by burke, Mar 7, 2017.

  1. burke
    Joined: May 2014
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    burke Junior Member

    At the final stage of building a composite balsa/FG skin panel on a male form. The last step was laminating resin with biax/mat on the balsa and then 10 oz cloth for a smooth surface to start the fairing. Ready for fairing and hand sanding before rollering gelcoat or paint.

    Should I use a final FILL coat of laminating or waxed resin on the cloth? If I use laminating resin, then use 05800 3M Lightweight poly Filler with a fairing board- won't the high points be scraped down to the laminating resin and clog the sandpaper?

    OR

    If I use a waxed finishing resin FILL coat on the cloth, when I sand it to remove the wax surface before applying the filler, the sandpaper won't reach down into the cloth weave depressions and hence not bond? The plus is, the resin will cure hard and not clog the sandpaper during the final sanding after the filler was applied.

    Which ever resin I use before the filler, if I paint this exposed cabin roof panel, should I apply a waxed resin coat over the fairing filler to "seal it"?

    Thanks for any advice.
     
  2. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Forget wax on anything but the final coat of any project, and that's only done occasionally (I'm not talking gel coat here). Adding wax creates added cost, time and hassle to the project.

    The Fairing compound needs the un-waxed surface to bond to, and it will allow the resin below it to cure.

    If you plan to gel coat the surface, then NEVER use wax prior to that step.
     
  3. burke
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    burke Junior Member

    OK. But if I put the fairing compound now on the unwaxed resin and spread it in the valleys, but no on the peaks- won't the hand sanded sandpaper quickly clog on the soft (unfilled) peaks?
     
  4. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    I'm no expert but I did just finish fairing and sanding a hard top I built using epoxy resin and 10oz along with 1708 biax.
    Wax is a bad thing in my mind if you want to have a good bond so as Ondarvr said, stay away from that at all costs.
    When you are applying fairing compound, whether it's stuff you've made with glass spheres or purchased (3M filler, etc.), you want to fill the valleys of course. However, depending on how fair the surface is to start with, you may also need to add a little more unless you plan to sand into your top layer of glass.

    If your fairing compound is any good it shouldn't clog up your paper once it's ready to sand.

    Good luck.
     
  5. burke
    Joined: May 2014
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    burke Junior Member

    Sorry, I'm unable to explain this better. The fairing compound won't clog the paper. When I spread the filler using a fairing stick, there will be peaks and valleys. I'm worried that the peaks maybe 15% of the 20 SF and will have ALL of the fairing compound scraped OFF showing only sticky laminating resin which will clog the paper.
     
  6. fly186
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    fly186 Junior Member

    Well, I guess I don't understand the "sticky laminating resin" part. Did your resin not cure properly?
     
  7. burke
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    burke Junior Member

    Oh OK. I'm using polyester lamination resin which will not cure hard, it will cure sticky- on purpose.

    You are using epoxy resin. Different
     
  8. fly186
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    fly186 Junior Member

    I haven't worked with polyester resin so I'm shooting in the dark here. Will it ever harden completely? Does it need to be "coated" with another layer of resin for that to happen? If so, I would make sure you add enough fairing compound to completely "hide" the laminating resin underneath even after you sand it.
    Hopefully someone with more experience than me will have some better advice for you.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Fairing compound made from un-waxed polyester certainly clogs paper.
     
  10. Sparky568
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    Sparky568 Junior Member

    Like Onadavr said. No wax poly resin for your final layup. I haven't used the 3M stuff you mentioned but I have used poly fair from Mertons in the past. Might be a cheaper alternative? But completely cover the unwaxed poly. I'd wait a few hours before sanding. If you are concerned about peaks, valleys or trowel marks in your fair coat you can feather it out with a good stiff brush or cut 1/2 the bristles off a chip brush.
     
  11. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    No, you don't want to be sanding into the laminate, and if there are a few high spots they will sand fine. Most general purpose polyester resins will cure relatively tack free after a day or two, and you will most likely be sanding it with 80 grit or coarser paper, so it shouldn't clog.

    And if you are sanding on a relatively hard to sand surface (the laminate) and an easy to sand surface (the fairing compound) side by side, it will be uneven, so for a good surface profile you should be sanding on just the fairing compound.
     

  12. burke
    Joined: May 2014
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    burke Junior Member

    Your last point was the key. I forgot that lesson I learned 50 years ago. If there are adjacent hard and soft surfaces, they won't sand flat. The soft will end up lower.

    How to evenly apply a thin skim coat of filler? Apply with a notched trowel to regulate the thickness applied- then flatten it with a smooth trowel?

    Thanks
     
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