As most who have used micro-balloons before know, the cure of resin, mixed with micro-balloons, goes through stages throughout its cure. At the middle stage, it has thickened, yet is still quite pliable. One can smooth it out and/or help it along towards finer contours. Just wet it (or your finger) slightly first, then smooth it out with your finger. (Many prefer to wear gloves.)
Once it cured, I was able to begin masking around the filler. Anytime I found a moment in-between other specific tasks as Tim was working, I would sand the filler a little at a time.
I have found that using different diameters of sanding drums greatly simplifies this task.
A narrow diameter drum gets into the tighter crevices, easily and uniformly rounding the fillet.
The masking tape reduces the chances of inadvertently "cutting-into" the sheeting. If you sand through the tape, remove and reapply another piece; then continue sanding.