OK, so used the MAS microballoons for the first time to fair out a goof on my part on the SUP.  I didn't get the sides and tailblock quite right, creating some hollows near the stern. So, I used some fillet mix to reinforce the corner on the outside, filling some of the hollow but wanted to fair it up into a nice curve with the rest of the sides.

The microballoons w/ some wood dust added made a very smooth compound.  Much nicer to work than the fillet mix.  However, I didn't count on the sag I got in the deeper part.  Boo.  I caught it at the gel stage and scraped the droopy part flat with the squeegee but will have to put in another layer.  I'm assuming I need to wait for it to set to what I call "al dente" (can just dent it with fingernail) before trying to refair.  I was hoping to do a hot coat to avoid the extra time of cure, sand, clean.  Has anybody out there "hot coated" with microballoons?  I'll make it a bit stiffer next time, anyway.