Your build is fascinating! I've made several foam/glass floats for outrigger canoes, same process you've been using, and I've been following your thread with great interest. I only have experience with System Three's two part WR-LPU. Better finish sprayed than brushed. Not sure I could get orange peel with it, but haven't tried since this is part of a watercraft. Their Quick-Fair product works really well to fair up troublesome spots.
In my usual dirtbag way, trying to save money, I did not use their epoxy primer. I unpleasantly discovered that it took forever for the yellow LPU to cover the pink color of the foam. I probably wasted money rather than saved money.
Looking forward to the rest of your build!
ETA: short of dragging the float over rocks, I don't get many scratches. Crosslinked LPU is pretty tough stuff.
I'd love to hear more on your experiences with the system 3 LPU. It had my interest perked previously but I've heard a lot about it being a bit finicky to lay down. That would be one coating I'd have not issue with spraying at home verse pretty much all the two part stuff which has isocynate.
I was going to experiment with mixing my own fairing compound ~1part cab soil, 3 parts micro ballons, and 1 part resin to see how that worked out. Otherwise I've heard good things on that quick fair.
Their primer is also of interest being a water born two part. I was going to check out some industrial primers of that sort as well.