Thanks guys.
I guess when I think about a CA hardening coat, I imagine a hard, shiny finish that is strong but does not blend well (aesthetically) with airplane. I am about to start my first composite plane assembly and am thinking about upcoming gaps to fill or repairs to make. The microballoons mixture seems to more closely resemble the material of the airplane, so in my mind it would be less noticeable when painting over. I also have to work against a weight limit requirement, and am starting off behind the 8-ball with an older design, so anything that gets covered with less coats of paint is a benefit.