When used correctly, epoxy can be an adhesive, a filler or a coating. The key is to match your formula to its intended use, then measure very carefully.
WHEN ROB MILLER JOINED ONE OF MY WEEKLONG CLASSES to build a 15' skiff of mahogany plywood and epoxy, he learned that epoxy could serve three different functions depending on how you mix and apply it. First, he used it as an adhesive for laminating and bonding that’s so strong it can bridge gaps up to about 1/8" with full strength. To fill a larger gap or radius an inside corner (called a fillet), he added a filler to the epoxy to get exactly the color and strength characteristics he needed. When the boat was structurally complete, he brushed or flowed epoxy on all exposed wood surfaces, forming a beautiful and virtually waterproof coating.
“This has to be one of the best-kept secrets in woodworking,” he said once the boat was done. “Why isn’t everyone using this stuff all the time?”
Learn to master epoxy, and you’re in a position to be in complete control of areas of woodworking that were formerly a matter of luck or assets. It simplifies complicated laminations for things like compound curved legs or spiral staircase railings, because you can control the pot life and open time through your choice of hardeners. Epoxy also requires far less clamping pressure than most other adhesives, so you can make those complex glue-ups with fewer or smaller clamps. Perhaps best of all, epoxy retains full strength across joints that fit less than perfectly, making it your best choice when things are a little dicey.
You can also strengthen cracked, weak, or punky wood with penetrating epoxy. It’s great for woodturning, or for repairing rotten wood window sills or the column bases. Knots, wormholes or small flaws in wood or joinery can be filled and hidden by mixing epoxy with sawdust from the wood itself, or highlighted by filling with a contrasting material. Brush or pour epoxy on the wood’s surface to reduce moisture cycling so it can’t dry out and check, or become soaked and rotten.
Epoxy can help you solve a lot of problems, but it’s not a panacea: Compared to common shop glue it’s expensive, and since it’s a two-part product it’s more complicated to mix and apply. The high-strength epoxy formulations you want for structural applications typically take up to eight hours to cure (at room temperature). It’s overkill for well-fitting dovetails or mortise-and-tenon joints, and way more trouble than is necessary for interior panels. Also, keep in mind that because it’s so strong and durable, epoxy can be difficult to sand and smooth. But when you need it, there’s no substitute.
A little science
There are thousands of epoxy formulations, each with a slightly different chemical structure, to meet specific needs. Some are designed for flexibility, others for stiffness. Some are made to work well in humid conditions, others in controlled industrial environments. Most offer you a selection of hardeners to help you control pot life and cure time depending on the ambient temperature.
As a rule of thumb, the longer the cure, the stronger the epoxy. For nonstructural repairs, quick-cure epoxies (cure times from five to 30 minutes at room temperature) work well. You can pick them up at hardware stores and home centers, but don’t count on them for serious holding power or for exterior use. For those, you need something with an eight-hour cure time. These longer-curing structural epoxies are commonly used to build airplanes, boats and race cars. They’re a little harder to find, but readily available at places that sell tools for serious woodworkers, at boating supply stores and online.
Epoxy is so strong because of the way the molecules are formed in the chemical reaction between the resin and the hardener. Mixing the resin and the hardener together makes new molecules and heat is given off in the process. These new epoxy molecules are long and complex, and join in an interlocking confusion. It’s like what happens when five or six flexible people play Twister. The long molecule chains wind and thread through the spaces between each other, making (at least on the molecular level) a sort of a woven mat of adhesive.
For this to work properly, the resin and hardener must be mixed in the correct ratio for the molecules to form. New molecules begin forming as soon as you start mixing; the cure is complete when all the resin and all of the hardener molecules are used. It’s not like polyester resin, which cures by adding a catalyst. With polyester, if you want to speed the cure time you just add more catalyst. But if you add more hardener to your epoxy, you’ll get a sticky goo. Measuring is crucial to epoxy success, and so is mixing.
Proper cross-linking can only occur when the batch is mixed well enough that all of the resin can find all of the hardener, allowing them to form those twisted interlocking chains. If you don’t mix thoroughly, epoxy won’t cure. My rule is to slowly count to 150 as I stir, or two minutes by the clock. If you’re adding fillers (more on them later), do so after mixing.
You can make small batches of epoxy by mixing by eye, but it’s always better to measure. Use a pair of sticks with equal markings, paper or plastic cups marked with gradations, or simply buy the plastic ketchup-bottle-style pumps sold by the epoxy company. Different brands have different ratios of resin to hardener, so measure according to the directions for your brand of epoxy, and remember that your margin of error is only about 10 percent. If you’re off more than that, the batch won’t cure.
The first step in repairing this spot of rot in an old maple floor is to vacuum away all the loose bits.
Pour a thin, warm epoxy into the rotten area. Keep your eye on it and add more as the epoxy flows into the rot. When the epoxy dries it’ll encapsulate the rot, sealing it from further moisture cycling and strengthening it slightly.
Temperature and cure time
After stirring, you’ll notice the mixing cup seems a little warm. The chemical reaction of cross-linking generates heat. And, like with maple syrup, epoxy gets thinner as it warms. Something else is happening as the temperature rises: The resin and hardener crosslink faster. And this generates more heat, causing the reaction to speed up even more. What happens next depends on how you manage the temperature.
Let’s say it’s 70 degrees in the shop and you’re mixing a general-purpose epoxy with an eight-hour cure at a room temperature of 77 degrees. You’ve mixed the correct amounts of resin and hardener and now have a plastic cup full of epoxy. Leave the epoxy in the cup after mixing, and in about five minutes the cup becomes uncomfortably warm. The epoxy is starting a pot-life “doom loop.” Confined to the cup, the epoxy gets hotter and hotter, and the reaction happens faster and faster. Too hot to hold, you put the cup on the bench and note that it’s starting to gel.
During the next minutes the stuff in the cup gets lumpier, and after about 10 minutes the smoke starts to rise. A confined pot of epoxy can get hot enough to melt or deform the cup or even start a fire (if it comes into contact with, say a solvent-soaked rag in the trash). Then suddenly, the epoxy forms a solid block in the cup, useless after something like 12 minutes. Though rock-hard, technically the epoxy still isn’t fully cured. Tests show it attains about 80 percent of its strength in 24 hours, and reaches full cure after about two weeks.
Now mix another batch, but this time brush or pour the epoxy onto the wood within three or four minutes. You’ll find that the epoxy warms up and thins out a bit, but it won’t set up any time soon. Out of the pot, the epoxy is still cross-linking, but it’s not doom-looping. The wood and the air keep the temperature down and keep the cross-linking under control. You have something like half an hour of open time before the glue starts to gel.
I’ve been careful to stipulate room temperature, because air temperature has a big effect on pot life, open time and curing time. If the air temperature in the shop is 10 degrees higher, you have half as much time. Pot life is about six minutes, cure time about four hours. When the shop temperature is 10 degrees lower, the times are doubled.
While you may have little control over the temperature, you can also manipulate cure time with your choice of hardener. Most brands offer a choice of “fast” and “slow,” and some have an even slower version for use in hot weather. Generally speaking, a fast hardener will give you 10-15 minutes of pot life at about 75 degrees, a slow one gives from 20 minutes to half an hour (read your brand’s spec sheet for exact times and temperatures).
Chose your hardener based on shop temperature, size of the workpiece, length of time it’ll take to get all your clamps in place, and how quickly you can work. If you’re not familiar with epoxy, start with the slow hardener – especially if your shop is 70 degrees or warmer. If it’s warmer than 80 degrees, you’ll need a hot-weather formulation to give adequate working time to get things aligned and clamped. If your shop is cooler than 60 degrees, go with the fast hardener, or you’ll have to wait two days for your epoxy to cure hard to the touch.
You can also mix hardeners within a brand system to customize pot life. If fast hardener forces you to work too quickly and slow hardener takes too long to cure, try mixing them half-and-half and see how that goes.
Fill the sealed area with another batch of epoxy mixed with filler. Use a putty knife to pack it well.
Stuck on You
Epoxy works so well as an adhesive because it’s thin enough to penetrate the wood. It only goes in to the depth of a few cells, but that’s enough for the crosslinked molecules to get tangled with those of the wood. Plus, epoxy is strong enough on its own to span small gaps in a joint with full strength. Imperfect joints are just as strong as perfect ones – perhaps even stronger. With really tight joints (and with too much clamping pressure), all the epoxy can squeeze out of the joint in the hours it takes to cure. So give yourself a little more play in your joints when using epoxy, and don’t crank up the clamps beyond what’s necessary to hold everything in place.
Another problem with joints occurs when the wood absorbs epoxy from the glue line, leaving the joint glue-starved. The degree to which the epoxy is absorbed depends on the species and the moisture content. You can prevent this by simply brushing out the epoxy and letting it sit for about 10 minutes. Then examine the joint with a light shining across it and look for dull spots. Add more epoxy and then close the joint.
Some other points to keep in mind when using epoxy as an adhesive are:
• For best results, apply epoxy to clean, freshly machined surfaces on both sides of a joint. In lieu of machining, sand with 80-grit paper.
• Plan on keeping the joint in clamps for at least eight hours – longer at lower temperatures.
• Epoxy will cure at low temperatures – even below freezing – but the cure is very slow. The biggest problem under these conditions is that the epoxy might conceivably drip out of a joint before it cures.
• Room-temperature-cured epoxies start getting rubbery at about 120 degrees and lose significant strength above about 140 degrees. They regain their strength as the temperature falls. When using epoxy for structural laminations that will be outdoors (boats, planes, arching roofs, etc.) keep the surface temperature down by painting the finished product a light color.
• Bond strength can be increased by adding cotton, linen or plastic fibers to the epoxy until the mixture has the consistency of ketchup. Avoid glue-starved joints by first coating both sides of the joint with unthickened epoxy and checking for dull spots. Then brush the fiber-enriched mixture onto one surface before closing the joint.
• Fibers add so much strength that the cured epoxy is very difficult to remove by sanding, scraping or planing. Make sure you wipe away any excess before it cures.
• Epoxy won’t stick to anything containing oil, wax or silicone; nor will it stick to many plastics. It also won’t stick to wood that’s been stained with an oil-based stain or has become contaminated, say, by machine oil. Clean a suspect surface with acetone.
• Wipe oily woods (such as teak) with acetone before applying epoxy.
Tough as it is, epoxy is difficult to smooth. Make your life easier by cleaning up all excess epoxy before it cures. Because it doesn’t shrink during cure, you needn’t leave any extra to sand away later.
It all adds up
Epoxy is a relatively friendly petrochemical. In its cured state it’s fairly benign, but you need to be careful around the uncured stuff, particularly the hardener.
Each contact with the hardener takes you one step closer to becoming sensitized, and when this happens you’ll find that being in the same room with the stuff sets off an ugly rash and days of painful itching. Some people work elbow-deep in epoxy for decades before it happens; for others it’s only a matter of months. The best thing to do is to minimize skin contact with the uncured resin, and wear a good respirator when sanding epoxy that’s not fully cured.
Vinegar is a solvent for uncured resin, as are denatured alcohol and acetone. All are fine for cleaning tools or surfaces, but not recommend for cleaning the skin. Because they break down not only the epoxy but also your skin, minute amounts of epoxy and solvent can enter your body as you scrub.
So keep it off your skin. Always wear disposable gloves when handling resin and hardener, and when mixing and applying epoxy. Be careful about wiping your eyes or nose, or pushing back your hair or glasses when you’re wearing gloves – they could have epoxy on them that you can’t feel. Change your gloves as soon as they get messed up. If you don’t wear eyeglasses, wear something over your eyes when mixing and brushing.
When you do get epoxy on your skin, wipe it off with a paper towel and wash thoroughly with soap and water. Hand cleaners like Gojo or Fast Orange work even better.
Most epoxy dealers offer a selection of fillers from dense and metal-like to lightweight and easy-to-sand. And besides what they offer, you can add just about anything that suits your fancy. Just avoid any substance that epoxy won’t stick to. Unlike polyester fillers, epoxy shrinks very little during cure, so what you see is what you get.
Keep these things in mind when using fillers with epoxies:
• Use sawdust from the workpiece to fill flaws or hide joinery mistakes.
• Use powdered graphite for a rich black inlay (also produces a very slippery and electrically conductive surface).
• Add powdered turquoise or other gemstones for inlays or to highlight a knot or small flaw.
• Put wax or Vaseline on a bolt and insert it into an oversized hole filled with epoxy mixed with high density filler to cast high-strength threads in wood.
• Use sawdust, silica powder, or other moderate density filler to fillet and strengthen corner joints. Fiberglass tape over the joint adds even more strength.
• Repair rot by removing all the bad wood and then sealing the surface with thin epoxy. Pack the void with epoxy mixed with a soft filler and sand to the desired contour to match moldings or decorative details.
• Epoxies premixed with fillers are more costly, but usually less wasteful than mixing your own.
• Always add fillers after mixing. Fillers increase the volume of the epoxy in the pot. Don’t mix too much.
• Most fillers will shorten the pot life by acting as insulators that retain the heat of the chemical reaction.
• Before leaving the work to cure, make sure your filled surfaces are smooth and clean. Blobs of epoxy on a surface make it very difficult to retain flatness while sanding.
To avoid drips and the resulting scraping and sanding, use a natural bristle brush for applying thin coats of epoxy. Apply subsequent coats before full cure for a strong chemical bond.
Shelf Life and Storage
Sealed from the air (or, more properly, from the moisture in the air), epoxy has an unlimited shelf life. Once you open it, you’ll find that the hardener gets darker with age. This doesn’t affect the strength of the bond, though the color of the cured epoxy may have a slightly redder hue.
Through changing temperatures during storage, some resins crystallize over time. If your resin gets cold, it can happen sooner rather than later. You’ll see white grains in the bottom of the bottle, much like what happens to honey. Before you use the epoxy, you need to get rid of the white stuff by warming the resin. Put the resin near some bright lights, set the bottle in a bath of hot water, or gently microwave for a few seconds with the cap off. You can use the epoxy when the crystals disappear.
Smooth as glass
A thick coating of epoxy over wood makes for a tough, resilient surface that is nearly waterproof. It protects the wood from rot and moisture cycling, and has the beauty and visual depth of a dozen coats of varnish.
Getting a perfectly smooth surface takes some skill and work. Horizontal surfaces are easy – just put on a thick coating and the epoxy will self-level. Vertical surfaces are more difficult because a thick coat of epoxy will invariably drip and run, and sanding or scraping the uneven mess is no small job. It’s easier to apply several thin coats, but this isn’t always advisable since most general purpose epoxies are pretty thick and viscous. You can thin down “regular” epoxy by warming it up a bit by putting the containers in front of a bright light or in a bath of warm water. This is fine for occasional use, but when I have a big coating job I buy an epoxy formulated specifically for coating. They’re thinner and have longer pot lives, so you can mix up a good big batch without having it go off before you can apply it.
Since a good coating job will require at least three or four coats of epoxy, you’ll get best results if you understand some of the chemistry of curing. The surface of a cured epoxy coating is slick and hard. In most epoxies a waxy by-product of curing called “blush” rises to the surface after about four hours of cure time (assuming “slow” hardener at room temperature). Another coat of epoxy won’t stick to a fully cured coat – it’s too slick. You must rough up the surface to give the next coat something to grab on to. Depending on the brand of epoxy you use and the conditions of your shop, the blush may be so severe as to instantly clog your sandpaper (cool humid conditions cause more blush). If so, you must wash the blush away. The easiest way is to wet-sand with a maroon Scotchbrite pad, making sure to wipe the surface with paper towels to remove the blush-contaminated water.
If you don’t let the epoxy cure fully, you can skip the sanding since you’ll get a chemical bond between the coats. And since you’ll apply the next coat before the blush forms, you’ll ace that problem as well. The only downside is that without testing equipment, you can’t know exactly when the epoxy is too far cured for a good bond. The easy way to deal with this is to keep a close eye on the coating and apply the next coat as soon as the first is hard enough to brush onto the previous coat without dragging and pulling. If you wait much longer than that, it may have cured too much. If this happens, wait until the epoxy cures fully, then sand for a mechanical bond.
Here are some things to remember when coating with epoxy:
• If the epoxy and the wood differ by more than about 10 degrees in temperature you’ll get bubbles in the surface.
• You can reduce bubbles by heating the epoxy coating with a heat gun or propane torch. Keep the heat source playing over the surface – you don’t want to increase the temperature by much.
• Get the epoxy sealer deep into rotted or punky wood by drilling several small holes.
• Encapsulate the wood – seal all surfaces – to reduce moisture cycling. Apply a minimum of three coats, more if sanding between coats.
• For the best-looking surface, apply as many coats as needed to get a perfectly smooth surface before applying a finish coat.
• When removing drips or unevenness in coatings, scrape first, then sand.
• Use a cheap bristle brush to apply the coating. Rollers make bubbles.
• Epoxy is cured when you it’s hard enough to sand without forming balls or rolls of rubbery epoxy.
• Epoxy has no UV filters – it must be painted or varnished when outdoors or it turns yellow in a matter of months.
• Paint and varnish last longer over epoxy-sealed wood, and refinishing is easier since you won’t have to go down to bare wood again.
Aimé Ontario Fraser is a woodworking instructor and author of the new book “Your First Workshop.” Her new shop is located in an old factory converted into artists’ lofts in Bridgeport, Conn.