I've done a lot of research on the subject over the years and dealing with amine blush is one of those kinds of problem where it's helpful to keep in mind the environmental contexts to which observations and recommendations are reported. For instance, about half the reports I’ve reviewed mention carbon dioxide as a factor, however these reports tend to come from industrial applications where environmental application conditions might vary widely from what you fill find in your home, garage, or shop. In an industrial scenario the chance of having an elevated CO2 levels is a much more likely scenario than you experiencing elevated CO2levels in your shop or home. I doubt many of us have a plan to monitor and adjust CO2 levels in our shop, so it doesn't serve much of a practical purpose to worry about C02 since we really can’t do much about it. Instead, it probably makes more sense to consider what we can do in our shops or homes to minimize the possibility of this phenomena.
I’ve compiled a list of tips and best practices from a myriad of sources. Now, keeping in mind that these tips are specifically for reducing the possibility of blush occurring, we have to balance when we might want to bend or break some of these tips to get the overall results were looking for. If you don’t experience blush in your finish and you’re otherwise happy with your results, then keep doing what you’re doing. Interestingly, many of these tips are things most of us are already doing, but this is a good checklist even for more experienced users. Some of these are “no brainers” while others may contradict common practice and “rodbuilding lore” that is not based in chemistry.
1) Make sure any water based color preservers are sufficiently dry. If they are not absolutely dry you run the risk of problems. If in doubt, wait longer for your CP to dry.
2) Premix the resin and hardener before dispensing each part, especially if it has turned white and was subsequently heated to return it to a clear state.
3) Keep all materials at or very close to 70 degrees F. Don’t preheat epoxy to thin it prior to mixing. Placing items of different temperatures in contact with each other encourages water vapor to condense from the air. Pre-heating epoxy in open air, especially on days with a high relative humidity, will introduce moisture vapor to the mix. If you feel your current epoxy is too thick at room temperature, then choose a different thinner epoxy.
4) Measure your epoxy by volume, not weight.
5) Don’t futz with the chemistry – resist the “rodbuilding lore” recommending adding drops of acetone or other thinners. Very simplistically, all adding acetone or any other “thinner” accomplishes is to make it harder for molecules of part A to find and match up with their part B counterparts. The linking of molecules is designed to happen in a predictable way over a specific time period and futzing with the chemistry alters the desired chemical reaction.
6) Inadequate or improper mixing – the single most noted cause of blush. Mix the epoxy thoroughly and completely. Rather than trying to explain this in words, just do exactly what is recommended in this FlexCoat video: [
www.youtube.com] Note at the beginning of the video Roger shows warming the epoxy, do this only if it’s cold for some reason and try not to go above 70F (see cautionary note about temperature in tip #3 above)
7) Induction time, a necessary but rarely if ever mentioned step in the process. From a chemical reaction standpoint, insufficient induction (sweat-in time) can cause a lot of problems. Many epoxy coatings require a specified induction time in order to fully “compatibilize” the epoxy resin and the hardener. This pre-reaction will reduce the likelihood of poor film appearance and poor coating performance. Lower temperatures and/or high humidity will generally require longer induction periods. Improperly inducted epoxies will exhibit a lower gloss than expected, yellowing becoming progressively worse over time, polyamide/amine blushing, reduced chemical resistance, and/or a softer film. It should be noted that the time given for induction to occur must be subtracted from the mixed epoxy’s “pot life.”
8) After applying epoxy, and once the “pot life” has expired and the mixture begins to set, maintain a consistent relative humidity below 85% and a minimum temperature of 70 degrees F for a period of three (3) days. You can reduce this time by 50% by increasing the temperature 10 degrees F to 80 F.
9) If you can, avoid busting bubbles with heat. Recalling tip #3, understand that a difference in epoxy temp compared to the air temp encourages water vapor condensation which leads to blush. If you have to use heat to bust bubbles, don’t do it where the relative humidity is high.
Again, keep in mind that these tips are specifically for reducing the possibility of blush occurring, we have to balance when we might want to bend or break some of these tips to get the overall results were looking for. If you don’t experience blush in your finish and you’re otherwise happy with your results, then keep doing what you’re doing.
