glassing plywood prior to hull installation

Discussion in 'Materials' started by scott2640, Dec 30, 2016.

  1. scott2640
    Joined: Dec 2016
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    scott2640 Junior Member

    As the title indicates, does anyone have any experience with glassing the individual plywood panels that have been cut to shape prior to actually installing them on the hull?

    Do they remain flexible enough to bend?

    I'll be using several layers of 3/8" ply (species TBD) and obviously only the final layer will be glassed.

    This is the method my designer is recommending. He then indicates to fill the screw holes with glass seam tape and epoxy and then fair the entire hull.

  2. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    That's what I have done. Be meticulous filling any screw holes and make sure you seal the plywood edges really well. Once the screws are in you'll be working more vertically so you'll use a thickened epoxy putty to fill the countersinks.

    Unthickened epoxy, which is what you'll use to seal the plywood is fairly runny. So you want to work on a horizontal surface whenever possible. I was taught on this site to apply three coats of resin, allowing enough time between coats so that the piece could be handled but no mare than 24-36 hours between coats. When epoxy is applied this way you get a "chemical bond" between coats not just a mechanical bond.

    I've mainly used short napped chemical resistant foam rollers to seal. If you're building a 41 you may as well look for deals on cases of these rollers as you're going to go through a lot of them. Disposable chip brushes too, a lot.

    As for the bending, you won't have any trouble. Epoxy is like plastic and when used to seal the film does flex pretty easily.

    Done carefully epoxy makes a great sealer. I end up with three seal coats, then a layer of fiberglass cloth (4), a couple of more coats of resin to fill the weave of the cloth (5,6), two coats of epoxy primer (7,8) and two or three coats of finish paint (9,10,11). See what I mean about the rollers?

    I've done ply and hardwoods this way and after 10 years have seen no rot or water intrusion anywhere on the boat.

    Good Luck,

  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Unless it's a panel, that will need to be removed or isn't going to need significant cutting, drilling, etc., then sure you can pre-sheath. There are occasions for both raw and precoated possibly skinned too. I try to use raw wood to wood contact surfaces for gluing, taping, more than modest machine work and if an adjoining piece or panel, is also getting sheathed.

    The chemical bond "window" is usually shorter than 24 - 36 hours, if you want to be on the safe side of the bond. I don't let room temperature cured bonds wait more than 12, if I can help it.

    If looking to seal a panel/area, apply a initial sealing coat, then a second coat, before the sheathing goes down, which will also apply the third coat, if the weave isn't filled. If filling the weave on subsequent coats, use a fairing compound, not straight epoxy (more economical). Use straight goo to fill the weave (adding a 4th coat).
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