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Thread: Thickened epoxy for plywood laps?

  1. #36
    Join Date
    Dec 2012
    Location
    Providence, RI USA
    Posts
    1,258

    Default Re: Thickened epoxy for plywood laps?

    I spoke to the folks at Jamestown Dist. about their Thixo cartridges, and they said it wasn't necessary to pre-coat with unthickened, then they said that you could do it for say, a beveled ply edge.

    I've always coated both surfaces with unthickened and then used thickened in the joint. But what Eric says above rings true. I think I won't coat the face of the ply anymore. It does seem to contribute to running.

    I love the stuff in the cartridges. I'd use it all the time, but the price is a bit dear.
    "near it, a small whale-boat, painted red and blue, the delight of the king's old age."

  2. #37
    Join Date
    Mar 2005
    Location
    Saco, ME
    Posts
    2,143

    Default Re: Thickened epoxy for plywood laps?

    My first build was a Shellback. I used S3 Gelmagic in the laps. I think I used different epoxies to experiment and learn since it was my first boat. But I know a couple laps have the Gelmagic. S3 said that there was no need to coat the end grain with unthickened, contrary to my training! So I put the prethickened, Gelmagic right to it on both sides of the lap joint. I sailed the boat hard for years and the joints were picture perfect when I sold it later.
    Clinton B. Chase
    Portland, Maine

    http://tinyurl.com/myboats

  3. #38
    Join Date
    Dec 2005
    Location
    Shoreline, Washington
    Posts
    2,245

    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Clinton B Chase View Post
    My first build was a Shellback. I used S3 Gelmagic in the laps. I think I used different epoxies to experiment and learn since it was my first boat. But I know a couple laps have the Gelmagic. S3 said that there was no need to coat the end grain with unthickened, contrary to my training! So I put the prethickened, Gelmagic right to it on both sides of the lap joint. I sailed the boat hard for years and the joints were picture perfect when I sold it later.
    Thanks (also Falcon),
    What I suspected. Does not directly address the question of doing the same thing with your own thickened epoxy goo. But tends to back up my idea that the latter is fine without precoating. Probably best to use a structural filler like silica, cell o fill or fibers, thickened just to the point of not sagging. No thicker. Which takes a little practice. Don't starve the joint, don't be ham handed or overly aggressive drawing the laps together.

    The line up of epoxy products continues to expand. I think the manufacturers ideas about their own products evolve. We all make assumptions, which may have some foundation, but are not always correct.

    PS - I also, near as I recall, did a little bit of everything planking up the 2nd 18 hull. It is difficult to resist the temptation of soaking exposed grain with unthickened product. Seems like reasonable insurance. Won't argue that there are situations when it is prudent and sensible. Pre-sealing adds considerably to the time and tedium of the process. How stupid if not necessary in this case.

  4. #39
    Join Date
    Jun 2008
    Location
    hood river oregon rgods4@msn.com
    Posts
    40

    Default Re: Thickened epoxy for plywood laps?

    my method.....Have all clamps, other materials ready to go. When epoxy goes, it goes pretty quick.
    In warm weather = put on unthickened while plank(s) are flat on floor/table/etc. Let it sit until it won't 'move' very easily at all. Add a thickened layer, mixed to about a peanut butter consistency. This layer wants to be laid on fairly thin. Let it sit until it too won't move. It'll begin to 'skin over', and now you know you're getting close. Keep track of all this, meaning you have to hang around, checking it. You do not want this part of the curing to "go off"....it'll be almost too hard to work with. You're ready when you can lightly push your finger on it and it doesn't do much. Then join parts. It'll squish out, but it won't go too far. Remember, warm(er) temps speed up epoxy curing. Just don't let it "go off", or "kick", or it's too late.
    cool temps.... same as above, but things take longer; "going off" is slower and you can catch it if you waited a 'little too long' and can work quickly enough.
    Remember, it's not the clamping force that makes the joint, it's the epoxy. Too much clamp pressure removes lots of the epoxy, thus leaving the definite possibility of a starved joint.
    You can also do this very same process with unthickened epoxy....painting on "wet on wet" as many coats as you wish. It will eventually reach a point where serious prodding with your finger won't show much movement, if any. Install it then. Even if it reaches the "fingerprint" stage (bare finger tip, pressed down firmly, will leave a finger print) it's still workable. Under firm clamping pressure, things won't move. And the epoxy will bond with itself. But wait too much more tham this, it'll be 'going off' on you. Then it's too late.
    Cardinal rule in mixing is to make sure your mix container keeps the epoxy 'shallow'. It'll take much longer to 'go off' than a container that has a deep mix of epoxy. Epoxy 'going off' gets quite hot! You'll feel it getting warm! and, it'll begin to congeal.
    I'm building a Simmons Sea Skiff 22 foot. My first build. My first use of epoxy. Have used lots of epoxy. All wood boat, all wood coated more than once all sides and edeges. A few times it went off. 95% of the time things went just fine. I learned as I went. A couple of times, on a bigger part, I had a helper come over and we both mixed up epoxy batches so things wouldn't sit tooo long and go off behind our back. We worked faster, no idle chit chat. Worked out fine.
    If in doubt, practice on some scrap first. Really helps ease the tension and relaxes the learning curve.

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