By Tom Pawlak —GBI Technical Advisor & Tim Atkinson — Senior Chemist
I recently built a double-ended paddle for my kayak. The blades were made of thin mahogany plywood coated with epoxy. I had coated all the paddle parts with two coats of epoxy the day before, and overnight a thin oil-like film had formed on the surface of the epoxy. This is amine blush. To ensure a good bond between the blade and the shaft, I removed the blush with water, dulled the surface with an abrasive pad, and dried the surface with paper towels. I’m confident using my new kayak paddle because the mating surfaces of the shaft and blade were properly prepared prior to bonding.
Amine blush, sometimes referred to as bloom, forms when amines in the hardener react with carbon dioxide and moisture in the air. We have found that the same components that promote our epoxy’s strength and toughness also contribute to the formation of blush. Balancing an epoxy’s physical strength, mechanical properties and handling characteristics is a difficult task when formulating for the marine market. A slight change in chemical formulation can have a dramatic affect on the overall characteristics. Components used in WEST SYSTEM® Epoxy have been carefully selected to achieve the long-term, high-strength performance WEST SYSTEM Epoxy is known for.
Blush is water soluble and can be removed with an abrasive pad and water, after the epoxy has cured hard. 3-M Scotch-Brite® General Purpose Hand Pads or similar abrasive scrunge pads work well for abrading and dulling the shiny surface. Detergents and solvents are not required or recommended for removing blush.
Blush tends to clog sandpaper. Cured epoxy coatings sand much better after they have been washed with water. If not dealt with properly, however, blush can cause adhesion problems with subsequent coatings applied over it. Amine blush can also interfere with the cure of some paints, varnishes, and all polyester gelcoats.
You can eliminate the inconvenience of washing and sanding between coats by applying subsequent coats of epoxy while the previous coat is still tacky to the touch. This window of opportunity for re-coating can vary from 15 minutes to several hours, depending on the hardener chosen and the working temperature. Careful consideration must be given in deciding whether re-coating without washing is appropriate when the epoxy has cured beyond the soft-gel stage. In warm, dry climates like Arizona it may be all right but in cool, moist climates it is not. Once the surface has cured hard to the touch, re-coating is not recommended, even in a dry climate, until the blush is removed and the surface is abraded. Amine blush is most evident when a coating cures while the temperature is dropping. Maintain a constant temperature in the work area as epoxy cures, to help minimize the potential for blush to develop.
We have found some ways to reduce the need for washing and sanding. For example, on wooden surfaces the first coat of epoxy is often allowed to cure overnight so the raised wood grain can be removed by sanding. This sanding operation can be eliminated if the raised wood fibers are dealt with while the first coat of epoxy is still tacky. Use a plastic squeegee as you would a scraper or trowel to flatten the raised grain back into the soft epoxy. Timing is critical, done too soon the grain may spring back, and done too late the hardened epoxy will not allow you to flatten the raised wood grain. The appropriate time to use the squeegee is when the surface is tacky to the touch, because the wood fuzzies will flatten easily and stay down. The surface can immediately be re-coated, saving time by eliminating the need to wash and sand before applying the second coat.
Release fabric can be used in some instances to reduce the amount of surface preparation necessary before re-coating. Apply and squeegee the release fabric over the surface of the uncured epoxy. After the epoxy has cured, remove the fabric and the surface will be free of blush and have a rough texture that needs no sanding. This method is used most often when laminating several layers of fabric that cannot be done in one session or in specific areas where something will later be bonded. The following tips can help minimize amine blush:
- Avoid working in very humid conditions. More blush forms when there is a lot of moisture in the air.
- Work above 65°F (18°C). The epoxy cures slower in cool temperatures and allows more blush to form.
- Use 205 Fast Hardener when the temperature is below 72°F (22°C). 205 can even be used in warmer temperatures if you don’t need a lot of working time. Remember, epoxy cures faster in warmer temperatures so its pot life will be reduced.
- Check resin-to-hardener ratios. Adding too much hardener not only increases the amount of blush, it compromises many of the epoxy’s physical properties.
- Stir the epoxy thoroughly to evenly disperse the hardener in the resin.
- Use 207 Special Clear Hardener for clear finishes. It has been specially formulated for clear coating applications and resists blushing very well.
This information is meant to take some of the mystique out of dealing with amine blush. Remember, when in doubt as to whether the epoxy has cured too far for re-coating, it is always better to be conservative. Allow the epoxy to fully cure and then wash and sand the surface before re-coating, rather than risk a possible failure. You’ll find that as you gain experience and confidence using epoxy, and understanding develops that allows you an economy of motion. Re-coating with epoxy while the previous coat is still tacky becomes second nature, because it saves labor and keeps the job moving along.