Whether you are bonding, fairing or applying reinforcements, the success of the application depends not only on the strength of the epoxy, but also on how well the epoxy adheres to the surface to which it is being applied. Unless you are bonding to partially cured epoxy, the strength of the bond relies on the epoxy's ability to mechanically "key" into the surface.
For good adhesion, bonding surfaces should be:
1. Clean - Bonding surfaces must be free of any contaminants such as grease, oil, wax or mold release. Clean contaminated surfaces with thinners, acetone or other appropriate solvent. Wipe the surface with paper towels before the solvent dries. Clean surfaces before sanding to avoid sanding the contaminant into the surface. Follow all safety precautions when working with solvents.
2. Dry - All bonding surfaces must be as dry as possible for good adhesion. If necessary, accelerate drying by warming the bonding surface with a hot air gun, hair dryer or heat lamp. Use fans to move the air in confined or enclosed spaces. Watch for condensation when working outdoors or whenever the temperature of the work environment changes.
3. Sanded - Sand smooth non-porous surfaces - thoroughly abrade the surface. 80-grit sandpaper will provide a good texture for the epoxy to "key" into. Be sure the surface to be bonded is solid. Remove any flaking, chalking, blistering, or old coating before sanding. Remove all dust after sanding.
Special preparation for various materials
Cure epoxy - Amine blush can appear as a wax-like film on cured epoxy surfaces. It is a byproduct of the curing process and may be more noticeable in cool, moist conditions. Amine blush can clog sandpaper and inhibit subsequent bonding, but it is water soluble and can easily be removed. It's a good idea to assume it has formed on any cured epoxy surface.
To remove blush, wash the surface with clean water (not solvent) and an abrasive pad such as a Scotch-Brite. Dry the surface with paper towels to remove the dissolved blush before it dries on the surface. Sand any remaining glossy areas with 80-grit sandpaper. Wet-sanding will also remove the amine blush. If a release fabric is applied over the surface of fresh epoxy, amine blush will be removed when the release fabric is peeled from the cured epoxy and no additional sanding is required.
Epoxy surfaces that are still tacky may be bonded to or coated with mixed resin/hardener without washing or sanding. Before applying coatings other than epoxy (paints, bottom paints, varnishes, gelcoats, etc), allow epoxy surfaces to cure fully, then wash and sand.
Hardwoods - Sand with 80-grit paper
Teak/oily woods - Wipe with acetone 15 minutes before coating. Solvent removes the oil at the surface and allows epoxy to penetrate. Be sure solvent has evaporated before coating.
Porous woods - No special preparation needed. If surface is burnished, possibly by dull planer or saw blades, sand with 80-grit to open pores. Remove dust.
Steel, lead - Remove contamination, sand or grind to bright metal, coat with epoxy then (wet) sand freshly applied epoxy into surface. Re-coat or bond after first coat gels.
Aluminium - Remove contamination, sand to a bright finish and prepare with aluminium etch before oxidation occurs.
Polyester (fibreglass) - Clean contamination with a silicone and wax remover such as Prep-Sol. Sand with 80-grit to a dull finish.
Plastic - Adhesion varies. If a plastic is impervious to solvents such as acetone, epoxy generally will not bond to it. Soft, flexible plastics such as polyethylene, polypropylene, nylon and Plexiglas fall into this category. Hard, rigid plastics such as PVC, ABS, and polycarbonate provide better adhesion with good surface preparation and adequate bonding area. After sanding, flame oxidizing (by quickly passing the flame of a propane torch over the surface without melting the plastic) can improve bonding in some plastics.
It's a good idea to conduct an adhesion test on any plastic (or any other material) that you are uncertain about.