Bonding and Clamping

The preferred method for most situations: it promotes maximum epoxy penetration into the bonding surface and prevents epoxy-starved joints.

1. Step 1: Wet out the bonding surfaces

   Apply a neat resin/hardener mixture — no fillers — to the surfaces to be joined. Use a disposable brush for small or tight areas; a foam roller or plastic spreader for larger ones.

   You can move to step two immediately, or any time before the wet-out coat reaches final cure.

2. Step 2: Apply thickened epoxy to one surface

   Stir the appropriate filler into the mixture until it is thick enough to bridge any gaps between the mating surfaces and prevent a resin-starved joint. Apply enough to one surface that a small amount squeezes out when the parts are brought together with a force equivalent to a firm hand grip.

   For most small bonding operations, add the filler to what is left of the batch you used for the wet-out. Mix enough resin/hardener for both steps, add the filler promptly, and allow for the shorter working life of a thickened mixture.

3. Step 3: Clamp the components

   Attach clamps as necessary to hold the components in place. Use just enough pressure to squeeze a small amount of epoxy from the joint — that bead is the evidence the epoxy is in contact with both mating surfaces.

   Avoid excessive clamping pressure, which can force all the epoxy out of the joint.

4. Step 4: Remove or shape the excess

   Deal with the squeeze-out as soon as the joint is clamped. A wooden mixing stick with one end sanded to a chisel edge is the ideal tool — or shape it into a fillet instead of removing it.

With epoxy, any method of clamping is suitable as long as the parts are held so that movement cannot occur. Common methods:

• Spring clamps
• C-clamps and adjustable bar clamps
• Heavy rubber bands cut from inner tubes
• Nylon-reinforced packaging tape
• Applying weights
• Vacuum bagging

When placing clamps near epoxy-covered areas, cover the clamp pads with duct tape, or put polyethylene sheeting or release fabric under them, so they do not bond to the surface. Staples, nails or drywall screws are often used where conventional clamps will not work — any fastener left in place should be a non-corroding alloy such as bronze. In some cases the thickened epoxy, or gravity, will hold parts in position without clamps at all.

Pre-coating porous surfaces with neat (unthickened) epoxy before applying a thickened adhesive or fairing compound improves adhesion: neat epoxy penetrates and keys into surface pores and end grain better than a thicker mixture.

A fillet is a cove-shaped application of thickened epoxy that bridges an inside corner joint. It increases the bonding area and acts as a structural brace — and every joint that will be covered with fibreglass cloth needs one to support the cloth around the inside corner.

The procedure is the same as normal bonding, except that instead of removing the squeezed-out thickened epoxy after clamping, you shape it. For larger fillets, add thickened mixture to the joint as soon as the bonding operation is complete and before the bonding mixture has fully cured — or any time after final cure, once the exposed epoxy in the fillet area has been sanded.

1. Step 1: Bond the parts

   Bond the parts using the two-step method above, clamping to a small squeeze-out along the joint line.

2. Step 2: Shape and smooth the fillet

   Draw a rounded filleting tool — a mixing stick will do — along the joint, dragging the excess material ahead of the tool and leaving a smooth cove bordered on each side by a clean margin. Use the excess that gathers outside the margin to re-fill any voids, and smooth the fillet until you are satisfied with its appearance.

   A mixing stick leaves a fillet of about 9.5 mm (3/8 in) radius. For larger fillets an 808 Plastic Squeegee, cut to shape or bent to the radius you want, works well. Apply additional thickened epoxy along the joint line with the rounded mixing stick, using enough to create the size of fillet you need.

   For long or repeated fillets, an empty caulking cartridge or a disposable piping bag lays a consistent bead — cut the tip to suit the fillet size. A heavy-duty sealable food bag with one corner cut off does the same job.

3. Step 3: Clean up outside the margin

   Clean up the remaining excess outside the margin with a sharpened mixing stick or a putty knife. Fibreglass cloth or tape may be applied over the fillet area before the fillet has cured, or after it is cured and sanded.

4. Step 4: Sand once fully cured

   Sand smooth with 80-grit paper after the fillet has fully cured. Wipe the surface clean of dust, then apply several coats of resin/hardener over the whole fillet area before final finishing.