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Thread: cpes

  1. #1
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    Default cpes

    Does sealing a new marine fir plywood hull with several coats of CPES prior to painting aide in reducing the amount of checking that occurs in fir plywood hulls?
    Thanks

  2. #2
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    Default Re: cpes

    Absolutely. Don't over-prep the surface at first, like don't surface fill or putty over screws. Get the CPES on but rather than several coats just keep going over till it won't soak up any more. Let it cure, light sand, and tack with acetone damped cloth. Then you can do what little surfacing and screw hole fillin as are needed with CPES well settled under.

    If the surface is really wonderful and no holes to fill, then you could hot coat the first layer over the CPES. Read the directions. Personally I am reluctant to hot coat but some who do it like the chemical bond.

    I've mentioned before two Glouster Gulls built side by side down here about 15 years ago or so. The only difference was one was CPES first the other not. Both spend winter and summer in the open. One checked after its first winter on the beach. The other is fine to this day, though I hear it was light sanded and repained a couple years ago. Just the paint. The CPES is fine.

    CPES will not make up for total trash plywood but it will make good exterior stand up better than the superb marine ply of twenty years ago ever could without CPES.

  3. #3
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    Default Re: cpes

    Thanks Ian..
    Jim

  4. #4
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    Default Re: cpes

    Although I'm a fan of CPES, I'm not sure to what extent it controls checking in marine fir ply when compared to coating with epoxy.

    Epoxy would add more weight than CPES, but I suspect that it would provide significantly more anti-checking as well. And the cost is probably similar, depending on how many coatings of which product are used.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  5. #5
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    Default Re: cpes

    Are we talking undiluted epoxy and no cloth?

  6. #6
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    Default Re: cpes

    Well, I am. If I mean glass and epoxy, I'll say so.

    Personally I wouldn't build anything from marine-grade fir plywood for this exact reason -- to keep it from checking you have to use either CPES (according to Ian) or epoxy and usually glass cloth. This negates any advantage in lower cost, plus adds greatly to the work.

    You could use thin coats of epoxy with a very light weight cloth 2oz or whatever, and minimize the cost and work involved.

    Sure wouldn't hurt to try Ian's plan of multiple coats of CPES, then hotcoat the final one with a good paint. If it checks later, you can sand the paint off and then use epoxy and glass.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  7. #7
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    Default Re: cpes

    I can assure you that in my experience, epoxy alone will moderate but not stop fir plywood from checking. I tried it.

    Seems a much better idea to use a higher grade of plywood.

  8. #8
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    Default Re: cpes

    I experiemented with epoxy (WEST) as a sealer under paint on plywood and varnish on mahogony and spruce. It tended to be brittle and very difficult to repair and maintain if the surface was damaged. Once I heard of CPES and tried it I was much happier. If the surface takes a hit that breaks the paint and sealer, the CPES is straihforward to repair with a bit of hand sanding and can be blended well.

    Even absent damage, WEST sealed plywood on my dory Leeward checked within five years and I went to glass set in epoxy. I've not seen checking in plywood sealed with CPES in the examples I know which are about 15 years now.

    In fairness to WEST, at the time I was playing with it as a coating in the early '70's I did not read any G-Bros instructions on thinning and everyone else I knew who was trying it as a covering did as I did, just mix and paint. I think that such a thick coat (comparitivly, as unthickened and unthinned WEST is seriously more viscous than CPES and puts down a relativly thick and brittle layer. CPES, on the other hand, is like using 50% thinned varnish as a sealer - it really gets in there. It's more like part of the wood.

    My habits now are to prime and seal with CPES under everything, paint or varnish or sikkens, except maybe spars. Though my experience of late with CPES sealed oars makes me think that it would be fine on spars finished bright.

    I've not used Smith's glue. I'm so used to WEST as a glue that I've been sticking with it as I think that for all but serious epoxy engineers and boatwrights, gaining experience with a small number of product lines and using them correctly trumps flitting from brand to brand. But for sealing, CPES has proven better than any alternatives I've tied. I use a stuff called Glu-vit for setting glass and for problem sealer places where I want its mildly elastic ability to enter and chase cracks. A remarkable goop.

    Others may make other choises with perfectly good results.

  9. #9
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    Default Re: cpes

    Thanks again. I will try the CPES under the paint.

  10. #10
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    Default Re: cpes

    Quote Originally Posted by jclays View Post
    Does sealing a new marine fir plywood hull with several coats of CPES prior to painting aide in reducing the amount of checking that occurs in fir plywood hulls?
    Thanks
    Fir plywood comes from the store with checks before you even touch it with anything. Not big gaping ones but they are there in abundance. By the time you figure in CPES, you have bought a better quality plywood for your small hull that will survive better with paint alone than fir ply will with CPES. If fir is all you can get then so be it and I am sure you have your reasons for choosing it but the economy of the material disappears immediately once checking or finishing becomes a concern.

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