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Thread: Thickened epoxy for plywood laps?

  1. #1
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    Default Thickened epoxy for plywood laps?

    Hi all,

    I'm working on my first boat, a Shellback dinghy, and I'm a little unsure of the way I'm using epoxy as an adhesive. (West System)

    The procedure I used for scarfing the planking stock was to wet both sides of the joint with unthickened epoxy, wait 10 mins, then recoat to replace anything that had soaked in. That worked pretty well because I was able to really build up the epoxy.

    For the planks though, because the joints are not horizontal (ie. the bevel of each plank is sloping towards the floor), I'm finding the epoxy all flows down towards the bottom edge of the lap and I'm concerned that I'm not keeping enough glue in the joint. (My bevels are a bit rough - I need some gap filling.)

    Do people ever use thickened epoxy for laps? The bottle recommends adding thickener for use as an adhesive, but neither the Oughtread book nor Eric Dow's Shellback manual mention this. I've just attached the first garboard plank, and I'm expecting this dripping epoxy problem is just going to get worse as I continue up the hull.

    Anyway, I'm really excited to be doing this project - every time I go to the shop I'm learning and practicing a new skill!
    Last edited by adamarthurryan; 09-20-2017 at 05:25 PM.

  2. #2
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    Default Re: Thickened epoxy for plywood laps?

    Yes, thicken your epoxy for the laps. Put it in a "pastry bag" .... fill up a sandwich bag, twist up the open part, cut a little bit off one of the corners, then squeeze a bead the length of your lap. (Search around the forum for a better description.)

    There's always some squeeze out. Clean as you go. Denatured alcohol on a rag works wonders for cleaning up epoxy before it kicks. It might take a few tries before you figure out just how thick/viscous you want the epoxy. I guarantee by your final lap that you'll be an expert.

    I'd wet the laps first with unthickened epoxy, then chase it with the thickened.

    Here's a description of various fillers you might use ... https://www.jamestowndistributors.co...t.do?docId=225
    Quote Originally Posted by James McMullen View Post
    Yeadon is right, of course.

  3. #3
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    Default Re: Thickened epoxy for plywood laps?

    Okay, that makes sense. Thanks Yeadon. That's what I'll do then.

    I maybe got so fixated on following the instructions that I let common sense take a back seat.

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    Default Re: Thickened epoxy for plywood laps?

    Adam, i mixed mine to about peanut butter consistency or just shy. Make sure you get the correct thickener ( I'm using a diff brand of goop so cant advise fully on West, but they're all pretty much the same except for ratios) I also taped either side of joints but Alcohol may be easier.
    You desperately need the Gougeon bible which is geared towards working with West products. If you google something like "Gougeon on working with wooden boats" or West products site you will find the free download. Def the ants pants for you.
    Last edited by andrewpatrol; 09-20-2017 at 10:39 PM.

  5. #5
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    Default Re: Thickened epoxy for plywood laps?

    There are different things that work. For me, I use only unthickened till the actual glue up. If you have uneven faying surfaces requiring build up with thickened googe, you'll make a hard spot on you panel.

    So start with a clean faying surface. For me ALL machine assisted methods I've tried - electric planer, router, various sander/grinders - have been abject failures. I don't know how to make a jig that will control the tool across a 4' wide plywood panel. So I stagger stack the panels. For example, getting an 12:1 bevel on 1/4" plywood means making a bevel that's 3" wide. If the panels are marked and stacked with 3" steps, then the panels total thick enough for easy well supported working. A sharp (SHARP!) plane is your friend. It should have a nice long body. 18" is nice. If you can find one low angle, so much the better. I got mine rusty and nasty at a junk shop.

    Really, a flat bevel is pretty basic and you really ought to master it.

    Assuming flat faying, have prepared the space for gluing. A long straight plan (like 14" or so) to push the sides of the panels against. Plastic down where the gluing will happen to keep the job from sticking to your shop. Provision to keep the panels from sliding apart - epoxy is slippery. Sometimes I just nail the panels in place but it's better to have end planks you can adjust and then nail down. And how to press the faying surfaces together. I put down plastic and then have a batch of long sand tubes to lay along the joint.

    So, all in place. Have both panels to be glued with faying surfaces up and something under the panels at those ends to elevate them 8". This will get your 12:1 bevel in the 1/4" plywood horizontal and you can spread unthickened epoxy with no fear. Once you've painted on enough that it won't soak any more, take your 8" supports away, turn one panel over and keep it up and away from the other for a little, and put slightly thickened epoxy on the up-facing faying surface. Bring them all together, block the ends so no sliding, and apply the weights along the joint.

    Walk away. Assuming you spread the plastic far enough, all will come apart the next day and you'll have a joint where you can bend the panels in a gentle bow with now hard spot.

    G'luck

  6. #6

    Default Re: Thickened epoxy for plywood laps?

    I thought all structural joints should be made with thickened epoxy?

    Consult the West System bible cited by another or the SystemThree bible.

    Unthickened epoxy is thixotropic, which is a fancy way of saying it runs like an escaped convict. If the surface ain't level and the wood does not soak it up, the epoxy will run. There are only three things for certain in life: death, taxes and epoxy running.

    Powdered silica will eliminate the running. Glass fibers will add tensile strength. Wood flour will bulk-out the mix and make it creamy and tasty like peanut butter. Use all three together.

    In this photo, both mating surfaces have been wet-out with un-thickened epoxy but one surface has been buttered up with silica/fibers/flour-thickened epoxy. This joint is ready for mating.


  7. #7
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    Default Re: Thickened epoxy for plywood laps?

    I use wood flour for thickening for most applications, including plank laps. People are going to think I'm a shill for Geoff Kerr, but I recommend watching the "How to build a Caledonia Yawl" video series on Off Center Harbor's website. It really goes into glued-lap construction, and you can see things that most people don't write about, like using lots of goop and getting tons of squeeze-out.

    As a first time builder, I found it invaluable.

    Good luck and post photos! The shellback is a cool boat!

    Mike
    "near it, a small whale-boat, painted red and blue, the delight of the king's old age."

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    Default Re: Thickened epoxy for plywood laps?

    Just finished gluing a little kid's boat a bit smaller than a shellback (just over 8' total length made from 4mm marine okoume ply). The sheerplanks were attached glued lap. My beveling was a bit rough also, but done with a hand plane followed by sandpaper, so no huge gaps. I had the boat upright and clamped on the planks in position, then put a strip of masking tape on the underside of the lap wherever there were no clamps. If you are attaching the planks with the boat upside down you'd have to crawl underneath to do this.

    Then I painted unthickened epoxy into the joint, followed by a bead of thickened epoxy into the crack. I used the "piping bag" technique and before cutting a corner of the plastic bag I reinforced the corner with gorilla tape (extra-thick duct tape). I'm having pretty good luck using talcum baby powder as a thickener. I took the masking tape off when the epoxy was still a bit tacky but no longer saggy. If you wait too long sometimes you get bits of masking tape stuck in solid epoxy. I flipped the boat and repeated the process on the outside but this time the gap underneath was already filled and I didn't need the masking tape to keep the epoxy from running through to the other side.

    The little boat seems pretty bombproof and it probably helped that I screwed the lapped plank both into the stems and the midship frame and filleted it into the little angle frames near the end. This means that the greatest unsupported span of the lap was about 3'.

    I scarfed the sheerplank and it was my first time doing this. I stacked the panels to make a ramp and used a , so the joint probably wasn't as close-fitting as it could have been. Before the plank went on a hard spot was noticeable at the scarf but despite the short, tubby boat's pronounced curvature, this is now barely noticeable. The perfect is the enemy of the good.

  9. #9
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    Default Re: Thickened epoxy for plywood laps?

    Yes, for the glue in the joint it can help to thicken the epoxy a little especially if the glue line is curing in a vertical position.

    But even then, not always. I made a solid spool to be keyed to the axel from the steering wheel for a new steering system on Granuaile. This was laminated up from 1"x1" maple glued together with unthickened epoxy. All the glue lines were along the grain and saturation past the first few cells of the wood was not on. To make eventual rounding to just under 6" diameter, I stacked the rows from the bottom up or top down 4 strips, 5 strips, 6 strips, 5 strips 4 strips. On the bench I secured three steps for each side to hold the lower three tiers in place while curing. Before coating the strips with epoxy I put duct tape face up into the form, overlaid a sheet of plastic, and then stacked, wrapping with the plastic and binding with duct tape.

    Worked a treat as each vertical glue line was only an inch long and really nothing ran out.

    For 12:1 scarf joints in plywood, I have done with no thickening and with just a little thickening. It's a pretty flat bevel and I put duct tape down on the back side of each feathered edge and the top of each bevel to keep epoxy from leaking onto the surface and making a spot hard to sand cleanly. On the lower end before putting the two sheets together I put a third strip of duct tape that will span the joint from tape on the back of the lower feathered edge to the tape on the end of the bevel for the other. This results in clean joints with little wasted epoxy. And it results in plywood planks that can be bent with absolutely no hard spot at the scarf.

    For 8:1 scarfs of replacement planking bits in my carvel boats - 1-1/2" to 2" planking, this happens in place. Once the fit is right, I soak unthickened into the faying surfaces and thicken just enough that the stuff does not all fall out while I jam the new bit into place. This is about the thickness of warm smooth peanut butter. It's significantly thinner than how you'd thicken for surfacing on a rough hull.

    G'luck

  10. #10
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    Default Re: Thickened epoxy for plywood laps?

    ...and you can "retro fit" epoxy into gaps later. It does not need to be all perfecty.

  11. #11
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    Default Re: Thickened epoxy for plywood laps?

    Thanks so much for all the feedback. I glued up another plank this week and used thickened epoxy, which stayed on the bevel better. I mixed it a bit thin, actually, so next time I'll use more thickener - I'm just using the West System-brand silica.

    I'm getting better with the bevel, the first one was all over the place, but the next one was pretty consistent. Somehow the angle wasn't quite right still, a bit of a gap all along the upper part of the lap on the inside of the hull... Thanks for the reassurance, wizbang, I'll be sure to go over and fill in any gaps later.

    I'm having such a good time with this project. It seems like every time I go to the shop I'm learning or practicing a new skill. I'm _starting_ to get a handle on sharping my planer blades - that's still a challenge and I'm sure it's holding me back some.

    I'll take a pic the next time I go in and attach it here!

  12. #12
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    Default Re: Thickened epoxy for plywood laps?

    Oh, and interesting to hear about the tape and pour technique, BOI. Never thought of that.

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    Default Re: Thickened epoxy for plywood laps?

    With five glued lap builds under my belt, I can say that adding a little wood flour to the epoxy is the answer. I always wipe up as much squeeze out as I can with an acetone moistened rag so it's less work later.
    I was born on a wooden boat that I built myself.

  14. #14

    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Rich Jones View Post
    With five glued lap builds under my belt, I can say that adding a little wood flour to the epoxy is the answer. I always wipe up as much squeeze out as I can with an acetone moistened rag so it's less work later.
    Have another joint ready go. Scrape the excess off with a putty knife and apply it to the next joint.

    An acetone-moistened rag is a fire hazard. Molotov cocktail? Avoid acetone. Acetone is toxic and even worse than death, acetone may leave residual unwanteds on the surface that can hinder future epoxy/coating bonds. Remember what Yoda said to Luke, "Solvents avoid young Skywalker."
    Last edited by Mo 'Poxy; 09-24-2017 at 11:51 PM.

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    Default Re: Thickened epoxy for plywood laps?

    ya, don't like acetone.

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    Default Re: Thickened epoxy for plywood laps?

    I use acetone for cleaning tools, but for excess epoxy I just scrape it and wipe onto a piece of scrap wood. I'm not bright finishing, thankfully, so I don't have to worry about a bit of extra epoxy on the hull as long as it's smooth.

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    Default Re: Thickened epoxy for plywood laps?

    When gluing with thickened epoxy, we try to put some masking tape on the planks adjacent to the joint, but a little bit away - to allow for the fillet, if you're having one. Just be sure to pull the tape while the epoxy mixture is still "green" so as not to trap the tape under epoxy. Makes for a nice, neat joint and easy clean-up.

    If you clean up your tools with white vinegar before it starts to set, you can then clean up the residue with soap and water. Works well.
    "The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails."
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    Default Re: Thickened epoxy for plywood laps?

    You can use vinegar to neutralize epoxy. Costs less than MEK or acetone about $3.50 a gallon at the supermarket. For bonding I like cotton flox added until mixture is about like peanut butter then silica until mixture stops running. Coat surfaces with neat epoxy before bonding as suggested.

  19. #19
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    Default Re: Thickened epoxy for plywood laps?

    I find it easier to clean up after the epoxy has cured with a heat gun and a sharp scraper.

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    Default Re: Thickened epoxy for plywood laps?

    consider using a more flexible epoxy when thickening it (or a bit if flex agent)

    note someone above said the epoxy was thixotrophic - actually it is non-thixotrophic until you start adding thickeners. Epoxy floor paints are non thixotrophic (so they self level - like raw marine epoxies0 - epoxy paints are thixotrophic so that you get equal film thickness at the top of a wall as at the bottom. Fumed silica is often used in the epoxy paints - not enough to thicken, but enough to provide some thixotrophic properties.

    paul oman
    progressive epoxy polymers inc

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    Default Re: Thickened epoxy for plywood laps?

    What about 5200 instead of epoxy?

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    Default Re: Thickened epoxy for plywood laps?

    I loathe 5200. Probably because I'm too messy and clean up of 5200 is a chore.
    I'm surprised no one mentioned the pre-thickened tubes of epoxy that self mix. (610 from west or Thixio from Jamestown) I used it with high school students while building lapstrake and it is a blessing. Yes, the expense is much higher, but low waste, consistent viscosity, and speed make up for the added cost in my opinion. It is a great product for the new boat builder.
    It is a staple in my shop now in addition to the multitude of other fillers that I still mix with epoxy when the job calls for it.
    IMHO

  23. #23
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    Default Re: Thickened epoxy for plywood laps?

    This, from Jim Ledger's long thread, is how it's done.


  24. #24
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    Default Re: Thickened epoxy for plywood laps?

    Lapped boats are engineered to move as the wood swells and dries. Epoxy is brittle, and doesn't move with the wood. 5200 retains some flexibility. If you don't rely on the wood swelling to make it water tight, then you rely on glue. If the epoxy cracks or breaks, you've lost your water-tight seal.

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    Default Re: Thickened epoxy for plywood laps?

    Tensile Elongation of West Systems least flexible epoxy is about 4%

    (I think this may be better than wood)

    http://www.westsystem.com/products/c...al-properties/


  26. #26
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    Default Re: Thickened epoxy for plywood laps?

    I should have been clearer. I use epoxy on plywood lap construction. I would agree with you on solid wood that there is movement and that could cause issue with the use of epoxy.

  27. #27
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    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Desert Rat View Post
    What about 5200 instead of epoxy?
    Devil Sperm

  28. #28
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    Default Re: Thickened epoxy for plywood laps?

    Even if we agree that plywood is more stable than real wood, plywood still has some swell and shrink. Further, lap construction is less rigid that taped panel construction. There will be more movement in a lapped ply boat. A more flexible glue should be more successful (and easier to repair...) than a more rigid glue for a clinker ply boat.

  29. #29
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    Default Re: Thickened epoxy for plywood laps?

    Gonna have to just agree to disagree on that I guess. (So many epoxied plywood lap boats out there.)

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    Default Re: Thickened epoxy for plywood laps?

    Ian - That is one crisp scarph!

  31. #31
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    Default Re: Thickened epoxy for plywood laps?

    Just to toss in a suggestion for those who are reading, but yet to choose their glue system for that particular job, at SCAMP Camp we've been using WEST 6/10. Thats a single cartridge pre thickened epoxy glue with a mixing nozzle on the front. It is used in a conventional sealant or construction adhesive "gun", and we find it a huge time saver. Its only used where there is a job large enough to use up a whole tube, say three SCAMPs having a plank on each, as the nozzles are throwaways after about 30 mins or so. But they're wonderful to use, no mixing, no spreading, the nozzle lays a perfect bead on the joint, no sag, and our test joints show very good strength.

    Its expensive, but when we're herding half a dozen builders through a 10 day build course, the timesaving is an advantage of much greater value than the relatively small extra cost to the boat.

    A small point here, I wont use any epoxy resin as a glue without the thickening agent, thats like the rebar in cement construction, epoxy is very brittle without it.

    John Welsford
    An expert is but a beginner with experience.

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    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Desert Rat View Post
    Even if we agree that plywood is more stable than real wood, plywood still has some swell and shrink. Further, lap construction is less rigid that taped panel construction. There will be more movement in a lapped ply boat. A more flexible glue should be more successful (and easier to repair...) than a more rigid glue for a clinker ply boat.
    I've built quite a number of glued lap plywood boats, probably 25 or so, and my customers many many times that. I've never seen a problem caused by the expansion of plywood due to moisture content variation in an epoxy glued lap joint. If at the design stage I'm concerned about the stress on the join, I'll specify a tape along the back of the joint, .

    John Welsford
    An expert is but a beginner with experience.

  33. #33
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    Default Re: Thickened epoxy for plywood laps?

    System Three has a similar product to the West 6/10. Thixotropic, can be had in cartridges with mixing nozzles, convenient, expensive. With these and other thick non sagging epoxy products, I don't believe one typically 'pre-coats' the surfaces, I did not.

    The question for the OP is not if he should use thickened epoxy for the laps (sure), but if he should bother precoating the laps with unthickened epoxy. Where beveling has opened up the grain on the lap, precoating seems reasonable, although I'm not sure it is necessary. Seems even less reason to precoat the face lamination of the overlapping plank. The thickened product tends to run and sag on the unthickened film.

    At this point epoxy glued ply lap has a solid track record.

    If anybody can school me on the functional difference between thixo products like System 3 Gel Magic or West 6/10, and how one would thicken standard resin and hardener with a structural bulking agent like silica, cel o fil, or fibers, or even wood flour - I would be very interested.

    Eric

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    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Eric Hvalsoe View Post
    System Three has a similar product to the West 6/10. Thixotropic, can be had in cartridges with mixing nozzles, convenient, expensive. With these and other thick non sagging epoxy products, I don't believe one typically 'pre-coats' the surfaces, I did not.

    The question for the OP is not if he should use thickened epoxy for the laps (sure), but if he should bother precoating the laps with unthickened epoxy. Where beveling has opened up the grain on the lap, precoating seems reasonable, although I'm not sure it is necessary. Seems even less reason to precoat the face lamination of the overlapping plank. The thickened product tends to run and sag on the unthickened film.

    At this point epoxy glued ply lap has a solid track record.

    If anybody can school me on the functional difference between thixo products like System 3 Gel Magic or West 6/10, and how one would thicken standard resin and hardener with a structural bulking agent like silica, cel o fil, or fibers, or even wood flour - I would be very interested.

    Eric
    I've always been of the school where pre-coating before using thickened epoxy is good to avoid starving the joint. Are people using the pre-thickened cartridge adhesives without pre-coating? Are there pitfalls?
    "The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails."
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  35. #35
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    Default Re: Thickened epoxy for plywood laps?

    Quote Originally Posted by Canoez View Post
    I've always been of the school where pre-coating before using thickened epoxy is good to avoid starving the joint. Are people using the pre-thickened cartridge adhesives without pre-coating? Are there pitfalls?
    Indeed, part of my question. I've not used the thixos in ply lap, I've used them in other applications. CLC uses only thickened epoxy in their lapstitch - but they don't bevel the lap. You do wonder about the exposed grain of the beveled ply. The conventional wisdom of presealing has been with us for decades.

    What do you say John?

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