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Working with Epoxy

Wonder glue or miserable failure? Unless you understand the properties of epoxy and mix it properly, you can get problems. It must be mixed in the right proportions (and each manufacturer’s proportions are different), cured at the right temperatures, and the work assembled under the right humidity for the best results.

What is Epoxy?

Most epoxies come as two-part adhesives, Part A and Part B—the resin with accelerators is in one can or tube, and the hardener in another. Mixing them together in specific ratios gives you a working epoxy. 

In general, the epoxy is colorless although some have a light sheen to them. After the epoxy resin is mixed with the hardener it usually becomes opaque, but that clears when the mixture has set up. The hardener, part B of the mixture, must be mixed with resin to effect a cure. The reaction creates cross-linked molecular chains that give epoxy its spectacular strength.

Do I need fast, moderate or slow hardener?

Most manufacturers make a “fast” hardener that sets up within ten minutes or so in 80 degree weather (slower in cooler temperatures), a “moderate” hardener used for larger jobs that might take 20 to 30 minutes to set up, and a “slow” hardener.

Typically, small jobs require the use of fast hardener, allowing you to put the parts together, clamp them, and walk away.  For jobs that require the application of large areas of epoxy, a moderate hardener allows you to coat the surface, apply the laminate and get the job set up before the epoxy starts to cure.

Mixing a moderate and fast hardener together to adjust the cure rate will not necessarily give you a linear difference in pot-life or cure time. You will need to experiment with fast and moderate hardeners to adjust the cure rate.

If the job starts to cure before all the materials are in place the best option is to remove the partially cured material, let the cure finish, sand everything back, and start over.

Surface Preparation

Before mixing epoxy, make sure that the surfaces are clean, free of dust and sanding residues, waxes or oils. Wipe the surfaces down with a solvent—if it beads up, it indicates that additional contaminants are present and the surface needs to be cleaned again.

Use 40-60 grit sandpaper on wood to roughen the surface for good bonding. Use 80-120 grit on fiberglass surfaces and make sure you wipe down with a solvent. Fiberglass Solvent Wash 202 from Interlux is good for fiberglass. If you are gluing Teak wipe the surface with solvent to remove the teak oil before applying epoxy.  If you want to bond metal, roughen the surface with 40-60 grit sandpaper or even a heavy- duty grinder. Epoxies do not penetrate metals and getting a strong bond is difficult.

Mixing

Before starting work determine how much mix you will need and mix only that amount. Mixing large batches often leads to tossing unused resin out, or it can lead to the resin setting up in the pot and generating a large amount of heat. In some cases the heat generated will melt the cup and has been known to start a fire. 

Always mix epoxy and hardener in the proportions suggested by the maker (which varies by manufacturer).  For example, Epiglass is mixed in the ratio of 4:1, and Protech Marine Epoxy is mixed in the ratio of 3:1.

Most manufacturers supply mini-pumps to go with their gallon or quart cans, but if you do not have a pump, use a measuring cup or a measuring stick dipped in a cup of resin measured against another stick dipped in a cup of hardener. Don’t use the same cup or stick to measure resin and hardener. Also, don’t mix products–resins and hardeners from different manufacturers have different properties and may not work together.

After pouring both resin and hardener into a cup, stir thoroughly for a few minutes to ensure that the two parts have mixed properly, scraping the bottom and corners of the cup. A thoroughly mixed resin will have a whitish tinge and be almost opaque. A poorly mixed resin will not set up properly and will have to be scraped off the job. Always mix the resin and hardener before adding any fillers. That way you will be sure that the epoxy is mixed properly.

Using Epoxy

If you are making a simple wood joint using epoxy, mix the epoxy and coat both surfaces, wait a minute or two to allow the glue to penetrate the wood and press the two surfaces together. You can clamp or glue and screw them, however, beware of clamping the joint too tightly. If you squeeze all the epoxy out of the joint, it is likely to fail. Before the epoxy has set up, wipe off any excess. After the epoxy has set, use a chisel to clean excess glue off the joint.

If your joint is a little sloppily, that’s ok, simply mix a little wood filler, fine sawdust or other filler into the epoxy and set the joint up in the usual way. That’s the beauty of epoxy. It fills small holes and still gives a strong joint. In fact, experiments have shown that in a soundly made joint, the wood fails before the epoxy lets go.

Next issue:  Curing, sanding and working safely with epoxy.

Roger Marshall is the North American Editor for the Yacht Report, former Technical Editor for Soundings, and a Director and past President of Boating Writers International.

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