Step-by Step Guide: Measuring and Mixing Epoxy Resins
Careful measuring and thorough mixing of epoxy resin and hardener are essential for the epoxy to cure properly. Whether you’re applying the epoxy mixture to wet out fibreglass, as a coating, or a casting, the following steps will ensure a controlled and thorough chemical transition to a high-strength epoxy solid.
Gather your equipment
- Eye protection
- Disposable latex or reusable rubber gloves
- Clean mixing cups (If you’re using recycled materials, avoid anything that held fats/oils, like butter or margarine containers)
- Clean mixing sticks
- Small kitchen-style digital scale (0.01 ounce) or disposable graduated cylinders (measuring cup)
Ensure you are always starting with clean cups and brushes to avoid any dirt or debris from falling into the resin mixture. Next, dispense the resin and hardener into a clean plastic, metal, or wax-free paper container. Don’t use glass or foam containers because of the danger of exothermic heat buildup. Do not adjust the epoxy cure time by altering the mix ratio. An accurate ratio is essential for epoxy to fully cure and develop its physical properties.
Choose your resin
For this demonstration, we are using our Super SAP CLR epoxy resin system from Entropy Resins. All of their retail formulas are two-part epoxy systems made up of a resin (Part A) and a hardener (Part B).
As with all epoxy resins, our systems are designed to work in a specific mix ratio between the two parts. Our CLR epoxy resin with CLF Fast Hardener works in a 100:47 mix ratio by weight or a 2:1 volume ratio.
Measure by weight or volume
The steps to measure by weight or volume differ slightly:
Measure by weight (using a digital scale)
- Begin by powering on your scale and choosing your unit of measurement. Most scales measure in either grams or ounces. Ensure that you have chosen a unit of measure that can help you determine 0.1 ounces or grams. If the quantity is too small for the scale to give an accurate measurement, it will be hard to get consistent mixtures. We recommend a minimum 1.47-ounce mixture (1-ounce resin to .47 ounces hardener).
- Place your mixing cup on the scale and reset the scale to zero by pressing the Tare or Zero Scale button.
- Pour your epoxy first and note the amount poured. (Example: 10 Ounces)
- Calculate the amount of hardener needed based on the amount of epoxy resin that you poured. (Example: 10 ounces of epoxy requires 4.7 ounces of hardener) This will give us a total combined weight of 14.7 ounces of mixed materials.
- Pour in your hardener, directly from the bottle to the mixtures. If your scale resets due to inactivity before you have a chance to complete the mixture, you’ll only need to measure out the hardener. Epoxy and hardener have different densities, which means that epoxy weighs more than the hardener. We recommend keeping a chart for quick reference to ensure that you make accurate mixtures.
- If you pour slightly above your target weight, don’t worry. As long as you are within the 5% maximum error margin specified for this resin system, you’ll be fine.
Measure by volume (with mixing cups)
- Determine the amount of mixed material that the application requires (as close as possible). If you require more epoxy resin, you can always mix a second batch to finish off the job. Resin is an expensive input for any application, so try and keep wastage to a minimum.
- Begin by pouring the epoxy amount your mixture requires into the graduated measuring cup. When pouring, be sure that the bottom of the curved liquid surface, called the meniscus, meets the line that you're pouring to. Note that warmer epoxy will lay flat so give it a moment to settle before calculating the amount of hardener required.
In this image, you can see the meniscus is the bottom of the curved liquid line created by the liquid surface.
- Next, pour in the required hardener to complete the mixture. Most of our ambient cure wet lay-up epoxy systems from Entropy are 2:1 by volume, so we will be pouring two parts epoxy to one part hardener. Always verify using the manufacturer's directions printed on the bottle.
Example: Our total material requirement is approximately 8-10 ounces of mixed epoxy/hardener. In this case, we’ll use 6 ounces of epoxy and 3 ounces of hardener, measured by volume, for a total of 9 ounces of mixed material. If more is required to complete the job, mix up a batch of 1.5 (minimum recommended mixture).
Mix the resin
Now you are ready to mix. When measuring by weight or volume (without pumps), you’ll need to measure Entropy resins and hardeners by weight or volume to achieve the correct ratio of 2-parts resin to 1-part hardener:
- VOLUME - To achieve the correct 2:1 mix ratio by volume, simply measure out 2-parts resin to 1 part hardener before mixing the components.
- WEIGHT - The exact weight measurement for these ratios is slightly different from the volume ratio due to resin and hardener density. See the hardener label or technical data sheet for the weight ratio of the resin and hardener combination you’re working with, and use a scale to measure the desired weight. Datasheets can be found in the “PDF” tab on each product’s description page on this website.
Here are the steps to mix the resin
- After the two parts are poured at the correct ratio, mix them together thoroughly for a full 2 - 3 minutes with a mixing stick. Mix longer for larger quantities.
- Be sure to scrape the sides, corners, and bottom of the container several times during mixing. This will ensure that all the hardener is thoroughly mixed with the epoxy and should prevent the resin from having an improper cure.
- Make sure to scrape both sides of the mixing cup also. If the mixture doesn't have a single consistency (streaks remain), continue mixing until fully blended.
Dispensing the resin
Here are the steps and tips for dispensing your mixed resin:
- Avoid applying epoxy resins for laminating and hot-coating outside in cold temperatures, on surfaces with condensation, or on high-humidity days. Ambient cure epoxies such as CLR and CCR by Entropy Resins are greatly affected by temperature and humidity
- Consider spreading a very thin layer of mixed epoxy with a squeegee first and then smoothing out the finish using a brush. This will spread the resin easier over larger areas.
- Start pouring or applying the epoxy immediately. The larger the quantity of mixed material in the cup the faster the Pot-life and working time.
- If more epoxy resin is required, or there are issues with settling, wait until the resin has begun to tack and apply a thin second coat or fill low spots using a new batch of resin (30 minutes after first application). For the best application, apply epoxy in a criss-cross pattern and then allow it to settle before it kicks.
DISPENSING WITH PUMPS
Resin and hardener dispensed at the wrong ratio is the source of most cure-related problems. To simplify dispensing and reduce the possibility of errors, use Entropy Pumps to meter the correct ratio of resin to hardener.
Pump two full pump strokes of resin for each full pump stroke of hardener. Depress each pump head fully and allow the head to rise completely back to the top before beginning the next stroke. Partial strokes will give the wrong ratio. Read the pump instructions before using the pumps.
Before you use the first pump-dispensed mixture on a project, verify the pumps are delivering the correct ratio by following the calibration process on the pump instructions. Recheck the ratio anytime you experience problems with curing.
Prepare surfaces for adhesion
For good secondary (mechanical) adhesion, bonding surfaces should be clean, dry, and sanded.
PRIMARY (CHEMICAL) BONDS
Primary, or chemical, bonds occur when you apply fresh epoxy over partially cured epoxy. Primary bonding relies on the chemical bonding of epoxy layers where resin and hardener molecules from the previous layer are still reacting and can therefore chemically react with the next epoxy layer. This allows all the epoxy layers to cure together and fuse into a single layer.No surface prep is needed when applying fresh epoxy over partially cured epoxy. But after an epoxy application cures, the window for chemical linking closes. At this point, you’ll need to prepare the surface for subsequent epoxy coats.
SECONDARY (MECHANICAL) BONDS
Secondary or mechanical bonds occur when you first apply epoxy to a substrate or apply subsequent coats over an epoxy coating that has fully cured. Secondary bonds rely on the epoxy’s ability to “key” into pores or scratches on the surface, so these bonds are mechanical in nature.
When you properly prepare the surface for a secondary (mechanical) bond, it needs to be clean and textured. This allows for good adhesion.
Sometimes you don’t want the epoxy to bond to a certain surface, such as a mould surface. Materials/substrates the epoxy does not adhere to are: clear cellophane tape, thin plastic film 3 to 5 mils, polyethylene plastic, and polypropylene plastic. You can also use mould release agents like paste wax, chemical release, and PVA (polyvinyl alcohol) to prevent bonding. To verify that the epoxy will release from the surface, we recommend trying it on a test area.
REMOVING AMINE BLUSH
Amine blush is a by-product of the epoxy curing process. This wax-like film may start to form during the tack-free stage of the initial cure phase. The blush is water-soluble and easy to remove but can clog sandpaper and inhibit subsequent bonding if not removed.
Simply wash the surface with clean water and an abrasive pad. We recommend 3-M Scotch-Brite™ 7447 General Purpose Hand Pads. Dry the surface with plain white paper towels to remove the dissolved blush before it dries on the surface. After you wash it with the abrasive pad, the surface should appear dull. Sand any remaining glossy areas with 80-grit sandpaper. Wet-sanding will also remove the amine blush.
Curing and epoxy clean up
The mixing of resin and hardener begins a chemical reaction that transforms the combined liquid into a solid. This period of transformation is called the cure time. As it cures, epoxy passes from the liquid state through a gel state before it reaches a solid state (Figure 1).
As it cures, mixed epoxy passes from a liquid state, through a gel state, to a solid-state. Cure time is shorter when the epoxy is warmer and longer when the epoxy is cooler.
Here are some terms you may see in curing instructions or packaging:
- LIQUID―OPEN TIME - This phase is also called working time or wet lay-up time. It is the portion of the cure time, after mixing, that the epoxy mixture remains a liquid and is workable. Do all assembly and clamping during the open time to ensure a dependable bond.
- GEL—INITIAL CURE PHASE - The epoxy passes into an initial cure phase when it begins to gel or “kick-off.” The epoxy is no longer workable and will progress from a tacky gel consistency to the firmness of hard rubber, which you will be able to dent with your thumbnail. The mixture will become tack-free about midway through the initial cure phase. While the epoxy is still tacky, about the tackiness of masking tape, you may still bond to or recoat the surface without surface preparation as the two layers will form a primary (chemical) bond. However, be sure it’s at the proper tackiness as the potential for a chemical bond diminishes as the mixture approaches the final cure phase.
- SOLID—FINAL CURE PHASE - The epoxy resin and hardener mixture has cured to a solid-state, and you can dry sand it. You will no longer be able to dent it with your thumbnail. At this point, the epoxy has reached most of its ultimate strength, so it’s fine to remove any clamps. A new application of epoxy will no longer be able to form a chemical (or primary) bond to it. Before applying more epoxy, you must clean, dry, and sand the surface to facilitate a good secondary (mechanical) bond. The mixture will continue to cure for the next several days to two weeks at room temperature (77°F or 22°C), becoming an inert plastic solid.
Here’s what you need to know about curing and cleaning up epoxy projects:
- Always stay with the project until it has begun to tack and cure.
- Allow the epoxy to cure at room temperature. If trimming is required, aim to cut or pull tape approximately 1-2 hours after the resin has begun to set. Trimming epoxy and fibreglass when 75% cured is much easier than once it has completely cured. Once fully cured, sanding and mechanical removal are required. Once dripping and running have ceased, edging tape can be removed.
- Clean-up using denatured alcohol, not acetone. Squeegees are easiest to clean once the epoxy is fully hardened (overnight). Brushes are single-use unless you have access to a freezer, in which case the brush can be kept for touch-ups. Beware that once it warms to room temperature, it will kick. If you get any on your skin, remove it immediately with a solution of warm soap and water.
The temperature will affect curing times. The cooler the temperature of curing epoxy, the slower it cures. Three things contribute to epoxy’s temperature:
- The general surrounding temperature of your work area (or ambient temperature).
- The temperature of the surface you’ve applied the epoxy to, which is also a part of ambient temperature.
- The heat the epoxy generates as it cures (exothermic heat)
To adapt to warmer temperatures, use a slower hardener to increase or maintain your open time. You can also mix smaller batches and use them up quickly. Or pour the epoxy mixture into a container with greater surface area (such as a roller pan) to allow exothermic heat to dissipate, extending open time. The sooner the mixture is transferred or applied (after thorough mixing), the longer the mixture is available for coating, lay-up, or assembly.
To adapt to cooler temperatures, use a faster hardener or use supplemental heat to raise the epoxy temperature above the hardener’s minimum recommended application temperature. Use a hot air gun, a heat lamp, or another heat source to warm the resin – before mixing or after the epoxy is applied. Do not heat hardeners prior to mixing with resins. You can speed the epoxy cure time by applying supplemental heat to the curing epoxy.
Top Coats and Finishing
The most important thing to consider when choosing a finish coating is protecting the epoxy from sunlight. Long-term UV protection depends on how well the finish coating retains its UV filters or pigments over the epoxy coating. A high-gloss finish reflects a higher proportion of the light hitting the surface than a dull surface. All other things being equal, a white (especially a glossy white) coating will last the longest.
Coating Compatibility : Most types of coatings are compatible with epoxy. Thoroughly cured epoxy is an almost completely inert, hard plastic. Most paint solvents will not soften, swell, or react with it. Epoxy amines can affect one-part polyurethanes and polyester gelcoat. If you’re using these, apply them after the epoxy is thoroughly cured, generally after two weeks at room temperature and after removing amine blush.
Elevated temperature post-curing will achieve a thorough cure much quicker. Post curing can also improve epoxy’s thermal properties and is recommended if you plan to apply dark paint over epoxy.
FINISH COATING TYPES
- Two-part linear polyurethane (LP) paints offer the most durable protection available. LPs are available as pigmented or clear coatings and offer excellent UV protection, gloss retention, abrasion resistance, plus compatibility with epoxy. However, compared to other types of coatings, they are expensive, require more skill to apply, and present a greater health hazard, especially when sprayed.
- Epoxy paints are available in one-part and two-part versions. Two-part epoxies offer many characteristics similar to higher-performance polyurethanes. They are durable and chemical resistant but offer limited UV protection compared to linear polyurethanes.
- One-part polyurethanes offer easy application, clean-up, and better UV resistance than alkyds. Be sure epoxy has cured thoroughly. Make a test panel to ensure compatibility.
- Alkyd finishes —enamel, alkyd enamel, marine enamel, acrylic enamel, alkyd modified epoxy, traditional varnish, and spar varnish—offer ease of application, low cost, low toxicity, and easy availability. However, they also have low UV and abrasion resistance. Make a test panel to ensure compatibility.
- Latex paints are largely compatible with epoxy, and they do an adequate job of protecting the epoxy coating from UV degradation.
Now you are ready to use your resin. Remember to check the safe use guides, safe handling information and technical data sheets that come with your epoxy system before mixing. For more information about the resins carried by Swell, browse our product catalogue for sea , snow, wood, and art projects.