Epoxy Basics
Stitch-and-glue boat building must be done with marine epoxy adhesives. Other resins such as polyester or vinylester are simply not strong enough to hold the panels together. You'll use epoxy (1) as glue, (2) as an adhesive coating when applying fiberglass tape and cloth, (3) as a sealant to protect and waterproof bare wood, and (4) as a gap filler and fairing compound. Several companies market extensive epoxy systems consisting of various resins, hardeners, and application tools. We've had particularly good results with System Three and MAS Epoxies.
Epoxy consists of two parts: the resin and a hardener, or catalyst, that's always mixed with the resin to cause it to cure, or set. The ratio of resin to hardener varies among brands of epoxy, so it's critical that epoxy is mixed precisely to the manufacturer's instructions. Epoxy that has too much, or too little, hardener will not achive full strength when cured.
Measure Your Epoxy - Since epoxy resin and hardener are thick and sticky, using cups to measure them is both inaccurate and unpleasant. Most epoxy manufacturers sell inexpensive plastic pumps designed to overcome the mixing problem (these are included in our MAS Economy Kits). These calibrated metering pumps screw into the tops of the resin and hardener containers and precisely meter the liquids. The epoxy and hardener will be dispensed at the exact ratio required and you'll only need to stir them thoroughly.
Remember, using a calibrated pump means that you have to push only one stroke for the resin and one stroke for the hardener. Do not push the resin and hardener pumps to the manufacturer's stated ratio, say, 2 pushes of resin to 1 push hardener if the manufacturer's ratio is 2:1. If mixing in a measuring cup you would use this ratio, but caliberated pumps will give you the right ratio when you push each pump one time. An incorrect ratio will result in the epoxy not curing. Also, increasing the amount of hardener will not result in a faster cure, but it will result in a much weaker epoxy.
Thicken Your Epoxy - Epoxy is thinner than most glues; it'll flow out of gaps and joints unless it's first mixed with a thickening agent, or filler. When reading instructions be sure to distinguish between hardener and thickener. Hardener, or catalyst, is always mixed into the resin; thickener is added only for some applications. Epoxy manufacturers sell many types of thickening agents for various applications. Silica powder (Cab-o-Sil) is a good choice for gluing. Wood flour or chopped fibers are used for making fillets and filling gaps. Micro-balloons or Micro-light make a lightweight and easily sanded filler for fairing and for fillets in racing boats. Thickeners do not have any chemical effect on the mix; they simply make the epoxy thicker.
The amount of thickener added to the epoxy varies with the application. When coating, sealing, or saturating wood with epoxy don't add any thickening powder. When using epoxy as a glue (i.e., joining wood to wood), add enough silica powder to the resin-hardener mix to bring it to the consistency of mayonnaise or jam. When filling gaps or making a fillet, add sufficient thickener, mainly wood flour, to make a peanut butter-thick paste. It is very important to mix the epoxy and hardener thoroughly before adding thickener, then to mix again after adding it. One epoxy manufacturer estimates that 90 percent of epoxy problems are caused by improper mixing.
Apply Your Epoxy - Since epoxy is readily absorbed by wood, it will affect the wood's bending properties. Wipe up any epoxy that oozes out of joints before it can harden. It's also important not to starve the joints: if too little epoxy is used it will all be absorbed by the wood, leaving a joint that is dry and weak. If a joint is to be clamped let the epoxy sit on the wood for a few minutes to determine if more is needed before clamping or joining the parts permanently. Don't apply any more clamping pressure than is needed to make a tight joint or you'll squeeze all the epoxy out. If you are re-coating cured epoxy, sand the surface lightly first. Also check for amine blush, a waxy film that forms on the surface of some epoxy; it must be removed (use soapy water and a scrub pad) before over coating to obtain a strong secondary bond. (MAS epoxy, if used with its slow hardener, does not usually blush.) It's best to overcoat within 30 hours as no surface preparation will then be required.
The minimum temperature for working with most epoxies is around 60 degrees, but if you require cure times of less than 24 hours (using "slow" hardeners) a temperature of over 70 degrees is needed. "Fast" hardeners are available for use in cool weather or for those of you in a hurry. MAS also offers a "Cool Cure" resin specifically for use in unheated, or under-heated shops.
As epoxy cures it generates heat, which accelerates the cure time. If a large mass of epoxy is left to cure in a container, such as a mixing cup, it will harden very rapidly, often in a matter of minutes. If, however, the same quantity of epoxy is quickly spread out, as when coating a boat's hull, it may take hours to harden. In the first instance there is relatively little surface area and the heat generated is not dissipated and causes the epoxy to rapidly solidify. Conversely, the thin film of epoxy, with much more surface area, allows the heat to dissipate. When long working times are important, always get the epoxy out of the mixing container quickly. Another trick is to pour epoxy into a shallow bowl or pie tin to increase working time. Containers of curing epoxy can generate a significant amount of heat; always place extra mixed epoxy outdoors.
In addition to the dispensing pumps and disposable mixing cups such as paper drink cups, yogurt containers, or clean tin cans, a few inexpensive accessories and supplies makes using epoxy systems much easier. You should have stirring sticks or popsicle sticks. For applying epoxy get disposable brushes, foam rollers, and a plastic squeegee or plastic putty knife. A soup spoon makes a good tool for applying fillets. When coating large flat areas use a no-lint foam roller (cut a full width roller cover in half and use them on a 3-inch roller frame).
Safety and Epoxy - Epoxy resins, hardeners, and solvents contain potentially dangerous chemicals. Avoid getting them on your skin; wear disposable gloves. You can clean tools and spills with acetone, but don't use it to remove epoxy from your skin. Instead, use soap and water, vinegar, or, best of all, mechanic's waterless hand cleaner. Unlike polyester or vinylester resins, epoxy has little odor, but you should still wear a respirator when sanding epoxy.
Take a look at Safe Boatbuilding by Dave Carnell for a more in depth look at how to keep yourself safe.
