ORIGINAL: Charlie P.
Some additives bulk up the epoxy for filling gaps, some strengthen it, some change the properties (for better or worse) and they can be abused to make the adhering properties worse. Add too many microballoons to a large amount of epoxy and you may have a melt-down thermal reaction.
As mentioned above: always mix the epoxy first. Let it stand briefly to link and then add the filler(s).
Great stuff, very informative.
This raises another question....
From that literature it appears that the cure time is determined entirely by the hardener, not the resin.
But what happens when you mix epoxy mixtures from different manufacturers?
I had an extra bottle of Great Planes hardener that I wanted to use with some Hobbico Resin.
I tested this mix, but the result did not look right.