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Working with epoxy is fairly straightforward, although there are occasionally a few issues one can run into. This paper briefly describes some of the common problems people have asked us about, and how to resolve them under most circumstances.
Please be aware that all resins & hardeners have their safety-related issues. Please research the products you intend to use and fully read the manufacturer’s safety information and follow their recommendations.
This paper is not intended to explain how to use epoxy, only as a guide on how to correct common problems when using epoxy. Please read carefully all manufacturer’s instructions and follow them carefully.
II. Types of Epoxy
Epoxies typically come in 2 parts (Resin and Hardener). Cheaper epoxies are often either thick and more difficult to apply easily to composite fabrics or will yellow when exposed to water, moisture, sun, or excessive heat (hereafter referred to as excessive conditions). Also, as a “general” rule, higher ratios of hardener are usually an indication of a weaker epoxy (so a 5/1 resin/hardener is “generally” stronger than a 3/1 or 1/1 epoxy ratio). Different epoxies have different strengths based on the curing/post-curing temp, so depending on your process, you may find your results vary from the manuf. specs.
It is difficult to determine the strength, stiffness, etc of one epoxy to another since the manuf. testing procedures vary. When choosing a resin, you may want to buy small quantities of a few types and make a few “identical” pieces, then test with your process before deciding on a specific epoxy.
Because of the high cost and time it takes to make composite products, we only use top-of-the-line epoxy. After extensive testing and research, we found the 820 epoxy to be the best epoxy in the market. Only several other epoxy brands did not yellow under the circumstances described above (excessive conditions), have very low VOC, have the temp capability of 215F+, don’t blush, are highly resilient, and are easy to work with for hand lay-up, vac bagging, and infusion.
Two-part epoxy resins can cure at different rates, based typically on the hardener. For example, the 820 Hardener comes in 822 (Fast), 823 (moderate), and 824 (slow). All the hardeners are UV stable, and thus will not yellow (as discussed above) except under extreme & prolonged UV exposure.
Many 100%, “clear” epoxies will yellow when exposed to these excessive conditions. The 820 resin is clear to nearly perfectly clear.
III. Applying Epoxy
Epoxy should always be applied very thinly (similar thickness as if you were applying sun-tan lotion [on your composite application]), not thickly like peanut butter. The only purpose of epoxy is to wet out the composite. That is, adding more or additional layers of epoxy will only add weight to your product, it will not add any additional strength! If you want a thick top coat buy a gel coat (this will eliminate any air bubble issues in your “top coat”).
Auto-measure pump kits will make your life much easier as well as provide you with accurate measurements every time. Incorrect amounts of resin & hardener can cause incomplete curing and/or too much heat build-up. Incomplete curing can cause a weaker final product. Excessive heat build-up can cause smoking or even fire in extreme cases.
IV. Common Problems and Resolutions
Below are some common problems people have asked us how to resolve. If you need further information, please contact us:
Epoxy turned cloudy when cured: This is commonly caused by too much moisture either in the air or moisture has gotten into the epoxy mixture. We have also seen this when the epoxy is heated with a heat gun and dried too quickly. Overmixing or overworking the epoxy may cause this as well.
Epoxy gets very hot: This is caused by either making too big of a pot (too much resin & hardener being mixed together at once, or too much hardener for the resin amount. Make sure your ratios of resin and hardener are correct. You can also mix this in a container that has more surface area (a bigger container). Adding more surface area to the epoxy while mixing will allow for the mixture to stay cooler. The epoxy should get warm, but never hot.
Bubbles form on the cured epoxy: A few bubbles are normal. This can be avoided by vac-bagging, or to some degree by covering the piece with thick plastic. Too many bubbles can be caused by the epoxy curing too quickly. Try curing at lower temperatures, or switch to a lower-temperature hardener. You can also break the bubbles as they form with a heat gun or a propane torch if you are not vac-bagging, but these are not orthodox procedures. Your best solution is to vac bag.
Epoxy yellows after cured when exposed to water, moisture, excessive heat, or sun:
Try using our 820 hardeners and add a clear coat with UV protection (such as a clear outdoor urethane).
Can’t get that professional shiny look: Spray the final piece with urethane, enamel, or other clear coats. Use 3-6 very very thin coats (to avoid sagging and dripping). You can also sand your part up to 5000grit sandpaper and then buff it to shine with some buffing compound found at any good auto parts store
If you have other problems, please feel free to contact us. We will be glad to help you!