INVISIBLE BUTT JOINTS
In 1978 I first used a plywood butt joint of
fiberglass cloth and epoxy resin to avoid having to make a scarph joint (not
easy and loses length)or use butt blocks (hard to work around). The joint is
so thin that careful feathering of the edges makes it invisible.
In 1986 I wrote about the joint in Small Boat
Journal. About the same time "Dynamite" Payson wrote in Boatbuilder
about a similar joint concept. Years later I discovered that Joe Dobler had
used the principle well before our publication, as had Jack Chippendale in
A piece of plywood bent around the side of a boat is
carrying most of the load in its outer and inner plies. The load (stress) on
the convex side (usually the outer) is a tension load trying to pull the wood
apart. On the concave side (usually the inner), the load is compressive-the
wood is being pushed together. The invisible butt joint makes two pieces of
plywood one by building a skin of fiberglass and epoxy on each side. When you
flex the joint, the load is carried entirely by those two skins you have
I made joints in various thicknesses of plywood and
tested them by breaking them in flexure with the maximum stress applied at the
joint. Joints that passed were ones where the plywood, not the
fiberglass-epoxy resin joint broke. My design basis for invisible joints in
plywood is: for 1/4" plywood, 1 layer of 6 oz. fiberglass cloth on each
side; for 3/8", two layers on the top (outside of bend) and one layer on
the bottom; for 1/2", three layers on top and two on the bottom; for
3/4", four layers on top and two on the bottom. Make the first fiberglass
strip on top 2" wide and each succeeding one an inch wider. On the bottom
side make the first strip 2" wide and the second one (if used) as wide as
the widest strip on the top side. After you lay up the joint cover it with a
piece of 4 mil polyethylene film and squeegee or roll it out. This presses the
cloth layers together and feathers out the excess epoxy onto the plywood. Peel
the film off after the joint cures and the surface is smooth and faired so
that very little filling or sanding is required. If you use woven tape instead
of pieces cut from cloth, the selvage may make a ridge at each side of the
joint. A joint with a single layer of 6 oz. cloth on each side is about 0.020
in. thicker than the plywood at its thickest point and tapers off to zero at
each side. Two layers on each side adds about 0.030 in. at the thickest point.
The joint in 3/4" plywood with four layers outside and two inside is only
about 0.045 in. thicker at its thickest point.
These joints are designed to use a minimum of material
to get the ultimate strength. I would only make them with epoxy resin (not
polyester resin) because: 1) epoxy bonds the glass cloth to the plywood in a
stronger joint that will not peel apart; 2) epoxy will always eventually
complete its cure; 3) there is no fire hazard with epoxy; and 4) there is less
of a toxic hazard with epoxy.
Originally, both Payson and I made the joint on one
side and turned the piece over to complete the joint. The turning over is
fraught with danger of destroying the joint that is very weak at that point.
I have gone to laying polyethylene film on a smooth
surface, laying the wetted out fiberglass tape (I use cloth to avoid the
selvage) on that, epoxy coating the face of the plywood that goes against
that, laying the plywood on the wet tape, filling any least void between the
plywood edges with thickened epoxy (this is critical, as any voids between the
butting plywood edges can make the joint weak), epoxy coating the upper
plywood joint surface, laying on fiberglass and wetting it out, covering with
poly film, laying on a smooth board, and weighting the assembly with concrete
blocks. In fact, the last time I did it I laid up a sandwich of two 16' by
20" pieces for the side planks of a sailing skiff and cured them all in
If you are making joints in plywood thicker than
-1/4", make the bottom side of the layup the one with the fewer number of
EPOXY IN A NUTSHELL
This is a distillation of my experience in using epoxy
for 30 years and improving my techniques. I started using epoxy for
boatbuilding in the 1960s. This was before Gougeon came out with their West®
system. I was using generic epoxy from Defender and an amine hardener that was
mixed 1:10 with the resin. Later I switched to Epon® resin and Versamid®
hardener from a surplus outfit in CA. This was a 2:1 mix and far easier to
Then, as now, all resins and hardeners were made by a
few major chemical companies. The companies selling products at retail develop
their formulations from commercial products.
Resin and hardener are ingredients that have to be
mixed in the correct proportion to cure to a solid with the desired strength
and hardness. If you want the mix to cure faster or slower, you pick a
different hardener. You don't change the mix ratio.
Epoxy is far superior to polyester resin because it
sticks to just about all materials, while polyester is not even a reliable
adhesive for laminating glass cloth to wood.
If you mix your epoxy in the correct ratio it will
eventually cure. If the catalyst you add to polyester does not kick it off, it
will never cure.
Epoxy resin and hardeners have shelf lives of many
years. I am still using a two-part surplus military epoxy putty that was
manufactured almost 25 years ago. The only exception to unlimited shelf life I
have found with epoxy resins is that the hardener for 1:1 mix systems thickens
and cannot be used after about a year.
GLUING & LAMINATING
The most important use of epoxy resin is as glue,
including gluing fiberglass to wood. Its advantage over most other glues is
that it will fill gaps; in fact, there always has to be some gap. If you clamp
too tightly the epoxy will be squeezed out so that the joint will be weak.
Adding filler to epoxy used as glue makes stronger joints, perhaps because the
filler keeps too much resin from squeezing out of the joint. One-inch boards
edge glued will break apart in the joint when it is flexed; add about 20%
pulverized limestone and the glued joint breaks in the wood. I add about 10%
limestone to resin when laminating fiberglass onto wood, also.
Fillers are added to epoxy resin to make putties for
two kinds of uses that have greatly different requirements. Those used for
structural joints alone or in combination with fiberglass should be as strong
as possible. Putties used for filling and fairing must sand easily.
The best filler for structural uses is pulverized
limestone (flour fine, not gritty as ground limestone is). It mixes to a putty
that doubles the resin volume and is dense and strong. It is universally
available as a fertilizer material at under a nickel a pound. It is difficult
to sand. Portland cement is pretty much equivalent. Talc, another mineral, is
almost as strong and sands easily. It also is thixotropic (the putty does not
flow, but will spread). It is available from fiberglass supply houses at
around a dollar a pound. For filling and fairing applications hollow bead type
fillers sand most easily because they are hollow and break. There are three
kinds of beads: thermoplastic (Microlight®) which can soften with heat;
phenolic, which are usually dark-colored; and glass (Scotchlite®), which are
white. The glass beads make the lowest density filler and are the lowest cost.
White wheat flour from the kitchen is a pretty good filler for finishing
A fiberglass-epoxy butt joint of plywood can be as
strong as the plywood itself. See "Invisible Butt Joints" above.
Right-angle joints in -1/4" plywood for rowing seat boxes, etc. can be
made with just a 1/4" radius bead of epoxy putty on the inside of the
joint. I tack such a box together with brass brads and then make the epoxy
fillet joints. For angle joints such as chines in -1/4" plywood a
1-1/2" fiberglass strip laid over a -1/4" radius epoxy fillet on the
inside and a 1-1/2" strip on the rounded outside edge gives a joint that
breaks by pulling the plywood apart. Bulkheads secured by a 1-1/2" glass
strip over a -1/4" radius epoxy fillet on each side fail in the plywood.
All joints must have the weave of the glass cloth filled smooth for maximum
strength. Many designs specify much more glass than needed. Make up short
specimens of your joints, cure them, and test them-in a vise, by standing or
jumping on them, or by running your truck over them. If the joint holds and
the material breaks, your joint is strong enough.
The principal hazard of working with epoxy resins is
from skin contact. The hardeners are the offenders. As a general rule, the
lower the mix ratio, the less the hazard (2:1 is less apt to irritate than
4:1), but you should avoid all skin contact and wash thoroughly after any
contact. Wash thoroughly before eating, drinking, or going to the bathroom.
Gloves and clothing help protect you, if they are clean.
You will note that I have not mentioned epoxy
encapsulation; i.e., coating both sides of everything with several coats of
epoxy resin. It has no advantages and is a waste of money and time that adds
useless weight. It won't turn lauan underlayment into marine plywood, though
it will make it cost nearly as much. It does not keep the water out of the
wood boat that lives in the water and a dry sailed boat doesn't need it.
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