cphanna wrote:
I'm back with a question that only occurred to me about ten minutes after my previous post. I realize many of you don't have the luxury of long waiting times between finish applications and so forth. But what about allowing a long curing and out-gassing period after applying epoxy? For example, if a hobby builder who wasn't on a schedule could hang the guitar up for a month or more before applying top coats of finish, do you folks think an oil varnish product like Tru Oil would be okay then? Or is this a more or less perpetual problem? Thanks in advance.
Link is on the money, Amine blush, which is actually hydrates of amine carbonate building upon the surface of the epoxy as it cures, is created as a result of an exchange between the amino acids (amines) contained in the hardener of 'most' epoxies, and the available CO2 and moisture in the air. The resulting 'blush' appears as an oily substance which can go completely unnoticed unless the cured epoxy surface is very carefully examined. No amount of waiting will remove amine blush, and sanding can quite often only result in spreading it over the work surface. Also, hydrates of amine carbonate will not wash from the surface using thinners or naptha and attempting to do so will mostly only move it from one place to the next on the work surface.
As i mentioned earlier, amine blush 'can' be washed from the surface of the work piece once it has fully cured using warm soapy water and an abrasive pad and then sanding before application of the finish. The reason I have suggested to simply avoid epoxy under any kind of polymerising finish such as Truoil is because the required process offers no real certainty that you will not encounter problems down the track. The odd thing with amine blush is that the finish can look and feel fantastic for a number of months and then the fuzz begins to appear to break your heart. The problem with this delayed reaction is that by this time it rears it's head, epoxy has fallen right off the radar and the finger usually gets pointed at the acids in the players sweat, poor quality finishing product or any number of other possibilities our minds can come up with. Often the only thing that will resolve the issue is to rub the finish back to bare wood and once that resolves the issue, what ever had been the assumed cause prior to work commencing, has now been 'proven' to be accurate regardless of how inaccurate it had been. The sad thing here is that no one learnt noth'in from the experience. Worse that that is the vindicated then run around telling everyone how bad this product was or that one was because they just do not understand what had really happened.
Anyhow enough of that rant. As to why some epoxies will blush and some not, you will find that this has to do with the 'type' of amines used in the manufacture of the hardener. Low blush epoxies generally cure softer and more pliable so they may not sand all that well. High blush products like Zpoxy cure to be more brittle, this is why they sand well. On that note it should be considered that there appears to be a relationship between contact dermatitis and high blush epoxy products. Some people who develop a sensitivity to epoxy can still tolerate working with low blush products like some of the West Systems lines but must avoid high blush products like the plague.
These factors aside, as Michael points out you can indeed use high blush epoxy such as Zpoxy and never encounter an amine issue. This is because in order for hydrates of amine carbonate to form during curing, both CO2 and moisture must be present in a high enough count to cause the exchange. So if you insist that you simply must use epoxy, the best bet would be to wait until the RH drops very low and be sure there is plenty of fresh oxygen rich air circulating around before you think about mixing part A with part B, or you could just fill with so many other products that it makes me wonder why anyone would bother using epoxy under a polymerising finish such as Truoil.
P.S. To be clear, just in case i was mis-read and it is assumed i had suggested that 'epoxy' gasses off, i had not. Rather i had suggested that 'Truoil' will continue to gas-off as it cures over a period of some weeks to months and this process could cause issue when reaction occurs between the surface of the finish and the synthetic micro fibers of a guitar case.
Cheers
Kim
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