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
England.
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 built.
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 one
operation.
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 fiberglass
strips.
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 use.
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 or talc and
the glued joint breaks in the wood. I add about 10%
limestone to resin when laminating fiberglass onto wood,
also.
FILLERS
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 a real bear to sand. Portland cement
is pretty much equivalent. Talc, another mineral, is
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 small amounts buy generic baby powder. Check that contents are talc and fragrance. 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 putties.
STRUCTURAL JOINTS
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.
SAFETY
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.
ENCAPSULATION
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.
