If you are going to use microballoons (I use them too from time to time), then it is best to mix them into a slow-curing epoxy. While the mixture does become thicker with the addition of microballoons, you still want it to remain loose enough to spread easily and to flow well. The need to roughen up the surface cannot be emphasized enough, though; you need to have the bonding surface well-structured to get the best results. I nearly always use slow-curing epoxy for everything now, the slower the better. Not only does it flow more completely into and around the joint and to level better, it is also easier to clean up any drips or excess because the mixture remains liquid for so long. The long cure process gives the epoxy time to penetrate into any porous material as well. I rarely use the fast curing stuff anymore for anything other than emergency field repairs, filling hollows and attaching nose weight. The biggest downside with using slow-curing epoxy is the need for extended clamp time. Since I am never under time pressure when I am building rockets, though, this doesn't present a problem for me.
Your LHS should have it.
