The all knowing all seeing Wikipedia explains 'Epoxy is a copolymer; that is, it is formed from two different chemicals. These are referred to as the "resin" and the "hardener".
The resin consists of monomers or short chain polymers with an epoxide group at either end. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A, though the latter may be replaced by similar chemicals. The hardener consists of polyamine monomers, for example Triethylenetetramine (TETA).
When these compounds are mixed together, the amine groups react with the epoxide groups to form a covalent bond. Each NH group can react with an epoxide group, so that the resulting polymer is heavily crosslinked, and is thus rigid and strong'.
Which Epoxy do I use?
In woodworking Epoxy is the choice of glue when gap filling is required. The most common wood glues are PVA like glues that require zero-clearance. These glues are not self supporting and any gap will lead to a tremendous reduction in bond strength. Epoxy left alone become a plastic like rigid structure can bridge gaps. Woodworking applications include to bond poorly fitting joints, filling defects in wood (checks, bore holes, splits), and useful on endgrain as it doesn't absorb along the fibres as other glues.
At present, West System Epoxy is my epoxy of choice. It is available from woodstores like Carbatec and boating and fiberglass stores. The cheapest supplier I've found is Fibreglass Material Services in Broadmeadow. West System Epoxy is a very low viscosity product (very thin and watery). They sell a variety of filler products like silica/microfibre powders that can be added allowing you to increase the viscosity to whatever the application demands.
Application 1: Fixing the Dust Extractor
A recent Dust Extractor mishap left the impeller housing split. After some encouragement (with a hammer) the housing went back together. To seal the joint and fill any gap I used epoxy mixed with iron oxide to thicken the glue. This will be then be sanded smooth and the paint job refreshed.
Application 2: Filling Wood Defects
Wood is not a homogeneous man-made product, but an organic (ex-)living tissue. Defects form by grow patterns (such as burls and branch points), insect attack (borer holes), and issues with drying (checking). For a consistent and stable finish these defects need to be filled. Here I am using West System epoxy with the silica/microfiber filler to fill gaps and stablise a burl. The excess can be scraped back and then sanded level with the wood.
This filler produces a clear/slightly white finish. Often it is more aesthetic to actually use a black filler. In the past I have used shoe polish and ebony sand dust. Iron oxide used to colour concrete is a cheaper and easier alternative. With shoe polish I noticed that the colour bled into the wood. To counter this you can use a shellac sealer to seal the wood prior to applying the epoxy.
Epoxy, O what a versatile glue...