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There are a million formulations of epoxy. The trick to using any of them is to know what conditions are required for a 100% cure of that system and what factors are detrimental to adhesion in any post processing of cured epoxy.
In commercial applications, composite fabricators often go to ridiculous lengths to remove the atmosphere, control pressure, and control temperature during the curing cycle. This is the point in building million dollar autoclaves. Once having fully controlled the process, epoxy:
1. Is totally inert, so it won’t shrink, gas out, or radically respond to temperature changes.
2. Has predictable mechanical properties…being homogenous.
3. Is versatile for use in many environments.
4. Is completely hydrophobic (most off the shelf brands).
5. Bonds like nothing else.
6. Is receptive to numerous other adhesives and paints.
A lot of changes have occurred over the years to commercial epoxies which have made them so much more accessible to retail customers. The chemical science of it has succeeded in removing many variables that add inconsistency to the final result, both in commercial and retail applications. Still, epoxy is what it is and million dollar autoclaves still exist to control variables. It goes to show you that careless or incomplete applications of epoxy will likely still cause problems, no matter what the state of the art.
So there are basic rules for the use of epoxy that, if followed, will produce very consistent results:
1. Stir it thoroughly and scrape the sides of the cup many times. “Thoroughly” to me means at least three minutes for about 50gms. Personally, to be sure about the mix, I sometimes transfer the mixed epoxy to a clean cup to guarantee that any unmixed material remains in the first cup.
2. Apply and cure in as dry a room as possible. Unmixed epoxy is mildly hygroscopic and will take in moisture which migrates to an exposed surface as it cures. The longer the cure cycle, the worse these characteristics (generally), so pick a cure cycle that allows you to accomplish your task and no more.
3. Avoid, as much as possible, an environment that is saturated with CO2. A dry, sealed room with lots of plants is probably the best practical situation. Don't choose to cure in a room with 40 visiting relatives....or in a pet store....or amongst heavy breathers in general.
4. Know exactly what the manufacturer recommends as a curing schedule and follow it closely. Each system is different but the temperature and time of the cure cycle of each system is of utmost importance to achieving an inert, cured state. Pad the recommendation by 50% if you are not 100% sure of these values. Epoxy can appear cured but still only be 70% fully cured. In this case you can count on further problems if you attempt to sand and apply a finish to it.
5. Most commercial epoxies have a published Tg (glass transition temperature). Retail epoxies should have this value published somewhere, if not on the package. This is important. Tg is basically the temperature above which cured epoxy becomes unstable. For most room temperature cure epoxies, Tg is well higher than room temperature but it’s not always much higher. It’s good to know at what temperature your glue bond WILL fail. It’s good to inform your customers. With other glues you might not know but with epoxy you can know.
6. Clean everything chemically prior to sanding and bonding as Kim suggests....but bond or paint as quickly as is reasonable after this prep work. Once you close off the access to CO2 the migration of oily residue to the exposed surface stops. Waiting too long might allow enough residue to the surface to cause bonding problems if your epoxy is not 100% cured. Of course this begs the point to understand and know if your epoxy is fully cured but there are a couple of situations in which “room temperature” epoxy never cures 100%....so perhaps it’s a good precaution to take.
_________________ I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.StuMusic
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