View Full Version : Invisible butt joints

09-26-2001, 11:11 PM
Dave Carnell describes a process for making "invisible butt joints" as an alterative to scarf joints when joining plywood. His web site is at http://home.att.net/~DaveCarnell/epoxy.html It seems like a very simple process. I would appreciate hearing from anyone who has tried this process. How strong are the joints? Have you had any problems with them?

Mike Field
09-26-2001, 11:49 PM
I've never tried it, but the principle is certainly sound. Assuming Dave's right with the number of layers, there's no reason why it wouldn't work satisfactorily that I can see.

Essentially, the plywood is just there to support the tape and epoxy -- at the location of the joint (although obviously not elsewhere,) it could be balsa, or paper, or anything else the same thickness, its job being only to support the tape / glue mix until cure.

Of course, the joint would be only truly invisible in the unlikely event that you could get the grain of the face-veneers to match on each side of the joint, or if you painted the whole sheet afterwards.

[This message has been edited by Mike Field (edited 09-27-2001).]

Wayne Jeffers
09-27-2001, 08:37 AM
I've noted the "invisible butt joint" with interest, too. One idea for possible improvement has occurred to me, although I have yet to try it:

Rather than simply butting the plywood, I suggest using a router with a 3/4-inch straight bit to cut a lap one-half the thickness of the plywood along the edges to be joined. The cut would be on the inside of one panel and the outside of the other and each would be just the width of the router bit. You would lose only 3/4-inch overall length from each joint.

In assembly, you could glue the laps, apply the glass and epoxy on one side, and allow to cure. Once the epoxy cures, it should be easy to turn the partially joined panels over to apply glass and epoxy to the remaining side without undue risk of damaging the joint.

One thing I've wondered about with Dave's design is to what degree it may be susceptible to failure from sheer loads. (Loads right at the joint, pushing in on one side of the joint and out on the other, as opposed to the compression/tension stress from bending the plywood.) It seems to me that the lap within the joint would also add a measure of resistance to sheer loads.

Like I said, I haven't tried this with an "invisible" joint yet. I have, however, cut laps exactly this way in plywood that I've joined with an old-fashioned plywood butt-block on the inside. I found the laps very quick and easy to cut.

Just a thought.


[This message has been edited by Wayne Jeffers (edited 09-27-2001).]

09-27-2001, 10:00 AM
Invisible? Of course, ...with filler and paint. I also found with 1/4" ply that it took a lot more plies than Dave specified to make the joint as strong as the wood. What I did might be overboard http://media5.hypernet.com/~dick/ubb/wink.gif but I replaced the face ply on both sides at the butt with glass (three plies) and feathered to none about 3" each side of the joint. To me it is a heck of a lot more work than scarfing and it takes twice as long. Glass one side, let cure, invert, glass the other, let cure. Two day job.

On the other hand...

...if Robyn can do it anybody can. http://media5.hypernet.com/~dick/ubb/smile.gif

[This message has been edited by NormMessinger (edited 09-27-2001).]

09-27-2001, 11:41 AM
Norm- Robyn owes you one for that. Remeber, pay backs are hell.

At one time visible butt joints were only a problem for male construction workers, but with the new "low rise" jeans the ladies are wearing, it seems to be a problem for both sexes. Not that I'm looking or anything.....

09-27-2001, 01:49 PM
been putting together panels for SWIFTWOOD's outrigger that way. Haven't had the time to put the hulls together yet.

I'll let you know once I do.

Dave Carnell
09-28-2001, 06:24 AM
Recently joined two 4' by 8' plywood sheets with one 2" wide fiberglass-epoxy joint in one operation.

Lay the first strip of glass wet with epocy on poly film. Lay the two pieces of plywood on top. Roll firmly. Make sure you have absolutely no voids (weak spots) between the two plywood edges by filling with thickened epoxy.

Then lay the second glass-epoxy strip atop the joint. Cover with poly, a smooth board, and weights. I have done two joints in narrower panels simultaneously as one layup sandwich.

Plywood panels in boats are most commonly loaded in bending. Shear loads across a joint are not likely in the completed assembly.

Wayne Jeffers
09-28-2001, 07:58 AM

Although I'm no engineer, I realize that the internal loading on plywood in boat construction is almost exclusively a bending load. My thought was that the modification I suggested might provide additional resistance to shear loads caused by an impact close on one side of the joint when the boat is in use. Of course, this modification would further ensure no voids (weak spots) between the plywood edges.

I wouldn't have suggested this except that I find this type of joint very fast and easy to make. I often use this type of joint where the ends of solid boards meet in my woodworking projects and even in a few carpentry projects, so I had practice before trying it in plywood for the first time.

Even if I were to glass both sides of the joint at one time, I think I would choose to lap the edges this way.


ken mcclure
09-28-2001, 10:54 AM
A friend of mine accidentally sat in a batch of CA glue (crazy glue) from a container that he knocked over as he was bending to sit. He jumped up quickly enough, and got his pants off quickly enough but alas! wound up with an invisible butt joint.

We immediately ran for the special glue remover....



Never mind.

capt jake
09-28-2001, 11:13 AM
Kinda sounds like a job for the Chemist!!
Did he jump around the shop doing the Funky Chicken?? http://media5.hypernet.com/~dick/ubb/smile.gif

09-28-2001, 11:48 AM
I donno Dave. I got it in my head the joint should be as strong as the rest of the strake. So my tests were to bend and twist. One ply on each side easily snapped at the joint with either a bend or a twist.


09-28-2001, 12:21 PM
I don't know why so many people have scarphobia. I have been scarfing full sheets of 6 mm Okoume for my deck skins. Probably done 120 lin. ft so far. With 6 mm just scribe a line 1 3/4" from the edge of the two sheets. Stack one on top of the other and line up the edge of the top one on the line of the bottom one. Use a power plane to bring it down close to the line and down to the bottom ply and finish off with 80 grit in the belt sander. Just keep the glue lines straight and even and they come out perfect every time.

With a little practice you can cut scarfs down the long side of a pair of 4x8 sheets in 10 minutes or less.

09-28-2001, 12:50 PM
In a no longer private email Dave asked:

How did you determine that one 2" wide fiberglass 6 oz. set in epoxy with on each side of ¼" plywood with no voids between the ends of the plywood pieces was not stronger than the plywood itself? I put my test pieces across a 2' span and loaded them at the joint with a jack measuring deflection vs. load until the piece failed, which was always outside the joint.
Dave Carnell

Well, it's been a while so I can't say how long the pieces I broke were, I'm thinking two or three feet and about three inches wide to approximate a strake. I just took them in hand and twisted or bent. When bending I attempted to bow the piece with my hands on the ends so as to distribute the tension evenly along the board. Not very scientific and I was probably biased going in. When bending a piece of plywood long ways the center ply adds little to the strength. The <3/16" face ply provides the tension and compression resistance. I couldn't see how a single layer of 6 oz cloth could provide as much resistance as the wood. Naturally, I proved myself correct. Happens all the time. Well, maybe not all the time.


I hope you don't mind keeping the discussion public, Dave.

09-28-2001, 10:46 PM
Just scarph the joint. A butt joint covered with layers of glass just won't be invisible.Besides, the scarph joint is much quicker than the glassed over butt joint,and easily as strong. The scarph joint is probably the easiest thing you'll do in the building process. If you can't do it (and I'm sure you can), you probably shouldn't be building a boat(and I'm sure you should.

10-01-2001, 02:42 AM
My 2 cents. I've butt-joined 10 mm okoumé plywood using 15 cm wide, 400 g/sq.m. biaxial tape outside and 300 g/sq.m. biaxial tape inside. I did the operation in one time, and it took about 20 minutes (not including cure time). Over-the-knee tests were passed with flying colors (stronger than a plywood butt joint with a backing plate). Critical points, IMHO:
1) before taping, the joint line must be filled with epoxy putty to avoid any trapped air inside
2)at least with thick plywood like mine, one should put a very substantial, evenly spread weight on top of the sandwich. I used about 60 kg on a 1-m long joint.
3)with relatively thick FG, the joint is visible and calls for a good fairing work.

10-03-2001, 12:57 PM
i agree with ppo. have done it also and had good results (snapped some leftover scrap - although could not do it over knee and bruised/scraped/cut myself pretty badly! - by jumping on it and it was the wood that failed, not the glue. fairing will definitely be your biggest task. having also made scarph joint i think that scarphing takes less time overall.