I sanded on the canopy skirt for about an hour tonight, making a big dusty mess of my garage:
After I did all the sanding I thought I could possibly do, I shot a coat of grey primer on the skirt.
Once the primer was dry (which was pretty quick since micro filler is so porous) I sanded the entire thing some more. The spots that remained grey are the low spots that I'll need to put more filler on. I'll have to do that later since I'm too tired to mess around with gooey stuff tonight.
By the way, I've become a convert to Norton 3X sandpaper. It's significantly more expensive than the no-name stuff, but cuts better and is way more durable. To me, anything that makes fiberglass sanding go quicker is worth it.