Epoxy: Workshop in a Can
Epoxy resin is an excellent moisture barrier coating
for wood or fiberglass; when it is used with the proper fillers, it's a versatile fairing
compound. It produces a strong, light laminate when used with fiberglass fabrics and makes
a fine adhesive for bonding wood, fiberglass, and metal. These characteristics - plus the fact
that doesn't rely on solvents or styrene to cure - mak it the first choice for do-it-yourself
Epoxy is a chemical product, and you must carefully follow all of the
manufacturer's instructions. Read the detailed product literature carefully before you start any
Epoxy is supplied in two parts. When the resin and hardener are mixed together in
a specific ration (usually 5, 4,or 3 to 1), an exothermic (heat-producing) reation takes place
and the mixture begins to cure. Most manufacturers sell inexpensive pre-calibrated pumps that
can be mounted on top of the resin and hardener cans to provide an accurate mixing ration.
Although manufacturers supply hardeners with different curing
characteristics, there are three basic cure phases; the actual time for each cure stage
depends on the type of hardener you use.
Open, or pot life (9 to 50 minutes).
When it's first mixed, epoxy is a syrupy liquid that can be brushed or rolled out, or mixed
with fillers or additives. Pot life will vary with the ambient temperature, type of hardener,
and the amount of epoxy that is mixed in the container. When you mix large quantities of epoxy,
more heat is generated, producing a shorter working time.
Initial, or green stage (6 to 24 hours).
Although it has not reached its final physical strength at this stage, the epoxy will no longer feel
sticky and can be indented with a thumbnail. A 'green" epoxy surface can still be shaped, but it is too
soft to sand. You can apply additional coatings of epoxy to the surface without having to
wash or sand between applications.
Final cure (24 hours to 9 days).
The epoxy has achieved 90 percent of its ultimate strength, and epoxy surfaces can be
dry-sanded. Clamps used in bonding applications can be safely removed. The surface must
be washed and sanded before any additional epoxy can be applied.
As epoxy cures, a waxlike film called amine blush, a by-product
of the curing process, amy form on the surface. If it's not removed before another
application of epoxy is added, it may degrade the bond between the two applications. Amine
blush is water soluble and can be removed by scrubbing the surface with water and a 3M
Scotch-Brite pad. Wipe the surface dry with white (plain) paper towels.
Working with Epoxy
The first step is to mix resin and hardener in a clean mixing
container; small 16- or 32-ounce plastic pots are ideal. It is best to mix small batches so
the epoxy doesn't begin to cure, or go off, before you're finished working with it. Use a
disposable stick - a wooden tongue depressor with one end cut square works well - to thoroughly
mix the resin and hardener together, scraping the sides and bottom of the container.
If it's not mixed thorough, the epoxy will not cure properly.
If you are adding fillers, do so after the resin and hardener have
been mixed together. Fillers are additives that modify the epoxy for use as a gap-filling
adhesive or fairing compound. Epoxy manufacturers supply a variety of specialty fillers,
ranging from low-density (low strength but easily sanded) to high-density (high strength
but very hard to sand). Blend the resin/hardener and filler until it has a uniform
consistency; you want a ketchup texture for laminating wood or gluing large surface areas,
a mayonnaise texture for general bonding or filleting, and a peanut-butter texture for
filleting, fairing, a gap-filling. Experimentation will quickly tell you how much filler to
Unthickened epoxy can be applied to almost any surface as a moisture
barrier or to provide a foundation for subsequent bonding or fairing applications. For
coating a small area, a disposable acid brush works well. For larger areas, use a foam-covered
roller and the same technique you would use for painting a wall with a roller.
After you have rolled on a coat of epoxy, drag a tip-off stick across the
surface with long, even strokes to smooth and level the epoxy. This will eliminate bubbles
and pools of epoxy, producing a surface that will require little or no sanding. To make
a tip-off stick cut a roller cover in half lengthwise pieces and then slice them into thirds
horizontally. Slide of of these pieces into a 45-degree slot cut in the end of a wooden
Unthickened epoxy can be mixed with fillers to creat an excellent
gap-filling adhesive. After the surfaces have been prepared and cleaned, thoroughly wet-out
all joints with unthickened epoxy. This is especially important if you are working with
porous wooden materials, as the wood grain tends to absorb the epoxy; by pre-coating, you
will avoid an epoxy-starved joing. Thicken the epoxy with filler to a ketchup
consistency, and apply it liverally to the bonding surfaces, enough so it squeezes out from
the joint. Scrape or wipe away the excess epoxy before it cures. Hold the parts in place
with clamps, weights, or staples; you need only enough pressure to position the pieces until
the epoxy cures; be careful not to 'glue-starve' the joint by squeezing all of the epoxy out.
Use epoxy and a low-density filler to level and smooth rough surfaces.
Trowel the epoxy mixture onto the surface, leaving it slightly higher than the surrounding
area. Use a mixing stick for small area; for larger areas, slowly drag a squeegee or batten
across the filler, bending it to the contour of the surrounding areas to evenly distribute
and shape the mixture. When filling a void that's more than 3/8-inch deep, apply the mixture
in two or more applications once the first layer has cured enough to support the weight of
When the filler mixture has fully cured, us a long sanding block and
50-grit paper to sand the surface. Change the sanding directon frequently and check the
surface for fairness with a batten. When the surface is close to final fairness, switch to
150-grit paper. Finally, apply several barrier coats of unthickened epoxy for moisture
protection and for a stable base on which to apply paint or gelcoat.
Working with fiberglass fabric is quite easy if you follow a few
simple procedures. There are two ways to wet-out fiberglass fabric. The first is to coat
the surface with unthickened epoxy, place the dry fabric over the wet epoxy, and wet-out
the fabric. The wet surface helps hold the cloth in place, but it can make it difficult
to reposition the fabric it it is slightly out of alignment. If you are glassing a large area, the
epoxy may start to go off before you have finished wetting-out and smooting all of the fabric.
The second way is to position the cloth over a dry surface, hoding it in
place with a couple of pieces of masking tape, and then wet-out the fabric. This method
makes it easier to reposition the fabric if you need to do so. When working with long
pieces of glass fabric - covering a dabintop, for example - this routine allows you to
wet-out the fabric in small batches to you don't have to worry about the epoxy going off
before you position the fabric.
Wet-out the fabric by drizzling zigzag patterns of epoxy on the cloth
in a small area, then work the epoxy into the fabric with a plaxtic squeegee. The cloth will
become translucent when it's thoroughly wetted-out. Do not leave too much epoxy on the
cloth, or else the cloth will bloat up from the surface, creating bubbles and voids
underneath it. Instead, squeegee off excess epoxy until the cloth has a matte
appearance and the weave of the cloth is clearly visible. Large glassing jobs always go
more smoothly when a helper can mix the epoxy as you wet-out the fabric.
Once the epoxy has cured to the "green" stage, fill the weave of
the cloth with several additional coats of unthickened epoxy.
Most fiberglas boats are built with a solid fiberglass laminate or
two fiberglass skins that are bonded to both sides of a core material, such as balsa, plywood
or foam. The hull may be a solid laminate, while the decks or cabintopy may be cored.
Solid-laminate repairs. If your hull
has been damaged by a grounding or collision, make sure the impact hasn't caused a delamination
within the solid-fiberglass lay-up. Drilling approximately 1-inc-diameter laminate samples
in suspect spots around the hull is a good way to test for any delamination. The bad news
is that you will have to repair these holes. The good news is that the procedure is the same
one you would use to repair any impact damage to solid fiberglass.
In the damaged areas remove all loose debris using a wire brush or
chisel to expose healthy, clean laminate all around. If the laminate has been holed
all the way through to the interior, first apply a backing piece to the inside of the hull.
To make the backing piece for the repair to the drilled hole, for example, cut a piece
of cloth 1 1/2 inches larger in diameter than the hole. Sand the hull around the hole,
then wet-out the hull surface 1 inch from the edge with unthickened epoxy. Wet-out the
piece of cloth and place it across the hole, smoothing excess epoxy from the fabric with a
sqeegee. Let the fabric patch cure overnight.
When the patch is in place and cured, build up the repair from the
outside. First grind a 12-to-1 bevel out from the center of the repair area. If the hull
laminate is 5/16-inch-thick, for example, grind out an area that is at least 3 3/4 inches
from the edge of the hull. The bevel is important because it allows plenty of bonding
surface for the epoxy and glass and helps to minimize point loading and stress concentrations,
which is what might occur if a plug were simply glued into the hole.
To fill the 1 1/8-inch beveled test hole, cut out circular pieces from
fiberglass fabric of the appropriate weight; in this case the diameter of the pieces in
the repair 'sandwich" should run from just over 1 1/8 inches up to 5 inches. The pieces of
fabric should increase in size from the inside of the bevel up to the outside dimension
of the uppermost layer. Use as many pieces of cloth as necessary to create a thickness that
is just under the total thickness of the laminate. The remaining space above the
sandwich can be filled with easily sanded epoxy fairing compount.
Sand the repair area, including the now-cured backing piece, and
wet-out the entire repair surface with unthickened epoxy. Wet-out the fabric disks on a sheet
of plastic, one on top of the next, beginning with the largest and ending with smallest.
Peel the wet sandwich patch off the plastic, and position it on the beveled area, with the
smallest disk toward the interior of the boat. Press the sandwich into place with a squeegee,
and remove excess epoxy. Finally, fair the repaired area with the hull exterior.
Cored-laminate repairs. Remove all
damaged skin and core to expose clean, solid material. If the core is sound but wet, dry it
with heat lamps or hot-air guns; the repair surfaces must be completely dry. If the
damaged area is less than 1 inch in diameter, it may be possible to simply clean out the
damaged core and fill the void with epoxy that has been thickened with fairing filler. For
larger repairs, replace the damaged core material with a similar product, cutting it to
fit snugly in the repair.
If the hole extends through both the inner and outer skins, you must
either apply a backing piece, just as with the solid-laminate repair, or repair the skins
from both inside and out after fixing any damage. If the inner skin is intact, remove all
the damaged core material and grind a 12-to-1 bevel in the outer skin.
Wet-out the interior surfaces of the repair hole and the surfaces
of the core material. Mix a batch of epoxy thickened with colloidal silica, and apply it
liberally to all bonding surfaces. Push the core material into place, making sure the
epoxy squeezes out eually all around the edge of the core.
Finish off the repair. With both types of repair, solid or cored,
work extra epoxy out of the cloth sandwich until the surface of the fabric begins to look
dull. Make sure the cloth doesn't have too much epoxy in it. Let the patch cure overnight.
The next day, scrub the repair area with a wet 3M Scotch-Brite pad
to remove any amine blush, and use a sander to knock down high spots and abrade the
surface. Wet-out the repair area with unthickened epoxy. Mix a batch of epoxy with low-
density filler thickened to a peanut-butter-like consistency. Trowel the mixture across
the repair so it is slightly higher than the hull surface around the repair.
Let the epoxy cure overnight, then sand the surface smooth. If
necessary, repeat the process until the surface area is fair and matches the contour of the
hull. After a final sanding, coat the repair with unthickened epoxy to seal the filler.
Let the repair cure for at least 24 hours before preparing the surface for painting.
Don't confuse polyester resin with epoxy. Polyester resin works well for
building a structure if all the layers of fabric, core material, and resin are assembled at
one time and allowed to cure together in a primary bond. However, using a polyester
resin to repair a cured polyester surface (secondary bond) can cause problems. Polyester
resin is not a strong achesive, and it may shrink as much as 8 percent during the
cure cycle. That kind of shrinkage can create stress concentrations and cause the
repair to fail.