The "Poor Man's Composite"
Or... How to build arbitrary structures cheaply, using
corrugated cardboard, hot glue, and a few simple
© 1991, 2004 by Steven K. Roberts
Nomadic Research Labs
Have you ever wanted to build a custom enclosure, oddly shaped
structure, waterproof canopy, camper shell, or special box -- only to
be frustrated by the cost and effort involved in fabrication? You know
the scenario: without facilities for this kind of work, you end up
spending far too much, compromising your design with scrounged junk, or
simply forgetting the whole damn thing.
This is the mode I was in back in 1990 when designing BEHEMOTH's trailer (or WASU,
for Wheeled Auxiliary Storage Unit). I knew what I wanted -- a
lightweight, waterproof, aerodynamic structure with wheel wells,
special flanges for equipment and antenna mounting, sealed access
panels, a 72-watt solar lid, and so on. But how would I build such a
thing? I even considered hacking up one of those hideous black plastic
tool bins they sell for pickup-truck beds.
I described the whole problem to David Berkstresser one evening. David
is one of those wizards who can sketch a dozen unique solutions to any
mechanical problem that seemed impossible only moments before, and is
the designer of the eccentric, agile, and swift Vacuum Velocipede
human-powered vehicle. He also did the CAD work on my folding console,
machined the Private Eye mounting, designed the new steering system,
and added various other artistic engineering touches to BEHEMOTH.
"Build it with cardboard," Dave told me, taking a sip of beer.
"No, seriously," I replied.
"I am serious," he insisted.
"Take a hot-glue gun, throw together your basic shape out of an old
refrigerator box, then fiberglass over it. I made a kayak out of
The elegance of his suggestion was instantly obvious. All sorts of
structures ranging from airplane wings to transit boxes for delicate
equipment are made of composites, structures that consist of two walls
separated by a "core." Usually some kind of foam or honeycomb, this is
what keeps the thin walls at a constant distance and provides
the dimensionality and moment that keeps them from collapsing. Some of
these materials, such as Hexcel, are among the most high-tech
structural materials available.
And garden-variety corrugated cardboard is a close analogue! It is used
everywhere you look for very good reasons: it's strong, light, and
cheap. Covered with fiberglass, it's waterproof, good-looking, and
stronger still. And best of all, it is infinitely hackable... if you
screw the design up, you can always scrounge more cardboard and brush
on some more goo.
Needing a name for this wonderful discovery, Dave and I puzzled for a
while and finally came up with CSPC... or Cellulose-core,
Silicon-matrix, Polyester-filled Composite. The results were beautiful:
I built BEHEMOTH
communication and power systems into a blazing-yellow custom trailer
that looks like a cross between a '56 Buick and a solar-panel-encrusted
satellite... and then later, used the same technique to build the
equipment enclosure behind the seat:
The first step in building anything using this method is to noodle over
it for awhile, sketching and perhaps even making models until you know
what you really want. Then buy a hot-glue gun and
lay in a good supply of cardboard.
A note here about materials. Corrugated cardboard comes in all sorts of
styles (never thought about this before, did you?), ranging from heavy
thick mushy stuff to thin, brittle material that cracks when you bend
it. Somewhere in between is the right stuff -- look for a good stiff
feel, thin walls, and consistent corrugations. There's no reason to go
with heavy cardboard for strength: all we're trying to do here is hold
the two fiberglass layers in a fixed relationship. Get material that
feels clean and light, and hasn't been walked on or creased.
Now you can start construction. Using a metal yardstick and a good
sharp knife (X-acto or retractible), cut the pieces as you need them,
being conscious of "grain" wherever you have to make a bend. At every
junction, run a bead of hot-melt glue, on both sides if necessary.
Don't let the glue glob up -- it's a pain to work lumps down to a
smooth curve that won't bubble the fiberglass.
There are a few tricks that make later work easier...
First, fiberglass doesn't like abrupt sharp angles -- it will gap and
leave ugly air spaces. Don't make any angles sharper than 45 degrees,
or any right angles with a bend radius of less than a half-inch or so.
You can usually get away with using a circular sander or rasp to
brutally round the edges, but if you can add a long strip to break
right-angle joints into a couple of more gentle bends you'll be better
sharp angles are unavoidable, you can lay the fiberglass cloth on the bias, with the weave
oriented at 45° to the edge around which you are trying to coax it.)
Gentle simple curves, like the front of my trailer, can be distributed
uniformly over a large surface by pre-bending the cardboard over a
table edge, parallel with the corrugations, one step at a time. By
carefully creasing the material at every wave, it will hold a smooth
curve -- though the inside surface will be rippled and cause lots of
tiny air gaps. It turns out that this doesn't matter much.
Dave swears that he has made compound curves by carefully analyzing the
corrugation pattern and making tiny, well-placed incisions with a sharp
knife. I find this astonishing, but knowing Dave, it's probably true.
Edges can be messy. They became in issue in two parts of the trailer:
the outside, exposed rim of the solar lid; and the edges of the long
stabilizing strips that add strength to walls. Details in a moment, but
be aware that every exposed edge will take extra effort and try to keep
them to a minimum.
Don't create any situations where you won't have easy access later
during the glassing process. Over time, you'll intuitively recognize
the behavior of the stuff, but for now just make everything as open as
possible. Really tight spots may be better approached using two or more
parts that are later joined -- it will keep resin off your elbows, and
probably look better.
Once you have the cardboard structure intact, stare at it, play with
it, and think through all the ramifications of the design. It will be
hackable later if you forget something, but it's a lot more so now. If
there's anyplace you will have a bolt compressing the surface, mash the
cardboard flat or hog it out and fill with cloth so it won't collapse
and loosen later; wherever there will be major stress, add
reinforcement. Smooth off all the blobs of glue, round the corners (I
did the outside edge of the solar lid by gluing on half-round pine,
sculpted smooth on the corners), and mentally run through the glassing
Like it? Time to make it permanent.
Establishing the shape of the wheel
Cloth and Goo
When you go out looking for fiberglass material, you'll find yourself
deluged with options (unless you go to the local hardware store, in
which case you'll have too few). I was in Silicon Valley at the time,
a local supplier with lots of stores whose personnel were quite
knowledgeable about the fiberglass cloth and
resins they sell. You can also buy retail from West Marine, or mail
order from the two biggies in the
Aircraft Supply and Aircraft Spruce & Specialty.
There are a
couple of basic choices you need to make at this stage...
First, polyester resin versus epoxy.
Polyester is cheaper and much less
nasty to work with (though it stinks more), but is less strong. Your
application will determine
which makes more sense -- although I used epoxies years later on the
Microships, for the less-critical bicycle projects I used polyester...
so that is what we will assume in this article. Also, be aware of the
meaning of "surface curing agent" -- a waxy additive that seals the
layup to help it cure. Problem is, it also interferes with adding
additional layers. Avoid this entirely, and use structural layup resin
or bond coat, not surfacing resin. My personal preference is the
thixotropic structural layup resin -- it's easy to brush on, doesn't
sag or puddle, and cures quickly. You'll also need catalyst, which is
the highly poisonous Methyl Ethyl Ketone Peroxide that should never,
ever, get in your eyes. And if weight is
critical, you can add millions of tiny air pockets by mixing "micro
balloons" with the resin.
Second, you'll need to select the
kind of cloth you want. This is very
important, for it will determine weight, flexibility, cost, and
appearance. It's also very much a function of the job. On the trailer,
I used Tap "A" and "C" weight cloth, the latter being lighter (5.85 oz
per square yard, .01" thick). Considering that your base material is
cardboard, going with super heavy cloth, thick mat, or woven roving is
generally overkill. At the other end of the spectrum is light "deck
cloth," which cures almost transparent. Again, think through the
project, decide where the stresses are and where you just want to
waterproof the surface, then choose accordingly. In my case, the
trailer floor and lid structures are mostly "A" cloth, and the
sidewalls are covered with "C."
Other materials for the job are mixing cups (start with the graduated
paper measuring cups, then once you get the hang of it you can switch
to old tin cans), stirring sticks, cheap brushes, acetone for cleanup,
paper towels, and scissors. You also need to decide how you want to
keep the stuff off your hands, and TAKE THIS ISSUE SERIOUSLY! These
chemicals have a cumulative effect when absorbed through the skin, and
you may get away with sloppiness for years then suddenly develop an
agonizing, lifelong allergic reaction to resin and catalyst. The
homebuilt aircraft world is full of sad stories about people who sold
their partially-built planes after careless habits made dealing with
When I started this project, I used rubber gloves and kept a bar of
Neutrogena handy. The gloves were loose and sloppy, and led to more of
a mess than doing it barehanded -- but it took a while to develop
techniques of dealing with the brush and mixing can (like always wiping
at the seam so you know where not to touch). After a few sessions, I
abandoned the sticky gloves and tried to be careful, washing after
every batch and using "Glove Cote," which is a lanolin-based cream
that's supposed to keep you safe. I'm careful anyway.
OK, ready to start? Take the phone off the hook. A batch under normal
thermal conditions will only last about 10-15 minutes, so you don't
want interruptions. Mix the stuff according to directions (around 5
drops of catalyst per ounce of resin, more if it's cold or you want a
fast cure) and lay down a preliminary coat on the cardboard surfaces
you'll be working on first -- this helps adhesion and is quite
necessary. When the batch starts to thicken, STOP -- it won't soak in,
and will have the opposite effect instead.
Now cut your first pieces of cloth and lay them on the cardboard, which
should be tacky. The wrinkles will brush out. Start laying up the
structure, adding layers where you think the major stresses will
concentrate (corners, support points, etc). This is where the art comes
in -- after a while you'll just know how much cloth to use and how
thickly to brush on the resin. In general, you want to use just enough
to make the cloth transparent, but not enough to puddle. The strength
lies in bonded cloth, not globs of brittle resin.
Take successive passes at the work, never mixing more than 5 ounces of
resin or so unless you're covering a large area with a large brush.
Overlap the cloth by a few inches, and keep working at it until you
feel that the coverage is complete. Notice the material's reluctance to
take sharp bends... this is how you gradually home in on the
constraints that affect the initial cardboard design. On exposed edges,
let the wetted cloth hang off -- don't try to wrap it around.
Finishing the edges is easy if you do this. After it's all dry, you can
slice the excess cloth with a sharp knife, then use a file to remove
any sharp points. It will look terrible. But now mix up a small batch
of Tap 500 plastic filler and work it into the exposed corrugations
with a plastic squeegee, then file and sand after it dries. The result
will be a smooth, waterproof edge that takes paint easily and
completely conceals the truth -- that the core of this beautiful
structure is an old cardboard box!
How pretty do you want this? If you've come this far, you'll have a
rather ugly brown material, with lots of ragged overlaps and texture
variations. If the need is purely structural, this may be the place to
There are two ways to add color. First, as I did on the trailer's
underside, you can mix special concentrated pigments with the resin (I
used black). This in no way alters the surface texture, but it does
hide the big Hotpoint logo which would otherwise show through the
The other way is with paint, and this is another one of those areas
where time, money, skill, experience, and luck all conspire to yield
either beauty or a giant mess. I did the trailer lid with yellow Copon
epoxy paint and had all sorts of problems -- now I need to sandblast it
off and try again. The body is another story: Maggie put her old
auto-body skills to work during a full week of applying Tap 500 (like
Bondo, but much more flexible) and sandpaper, finally perfecting the
surface with glazing putty. We then took it to Charles Tripp in Los
Gatos, who did a beautiful job of spraying on DuPont Imron (a fiesty
thoroughbred of paints, not for the poor or fainthearted). The stuff is
over $100 a gallon and can cause respiratory failure if you spray
without breathing apparatus, but it looks great. It's pretty, but is
still a bit fragile when abused... and inside, where there is no
bodywork, adhesive-based cable tie downs can easily detach a circle of
And there you have it. The trailer, for a moderate amount of time and
money, looks professional enough to prompt people to ask where I bought
it. I grin and tell them it's just some old cardboard boxes, glass,
goo, and Imron paint, all mounted on a custom chrome-moly frame built
by Rock Lobster and painted by Dr. Deltron. They don't believe me.
I do have one final suggestion if you try this on your own structural
fabrication problems. Do an unimportant test project first to get most
of the mistakes out of the way. That first layup will seem awkward and
messy, with the cloth buckling and resin dripping down your chin. But
give it a chance... it works! And when you're ready to move up to
higher-tech materials, you might enjoy Bob Stuart's very informative
discussion of designing
with Coroplast, as well as my own introduction to foam-core
The BEHEMOTH trailer complete and ready for communication system integration