Ground out the dings and divots I missed the first time, and feathered the edges of the paint around them.
Then I sanded the whole thing down again, this time using 150 grit.
The black paint makes a good "guide coat", easily showing the high spots where the paint sanded off, and the low spots, where it didn't. And wonder-of-wonders, it's almost acceptably smooth. This stuff sands off relatively easy with 150 grit, and I probably would have blown right through it with 80 grit.
I'll go over it again tomorrow when I have more light coming in. I have some "High Build Primer", but it doesn't specifically say it's for flexible parts, so I'm loathe to use it. The "recommended" primer should be here Monday, so I just have to sit on my hands for a while. This is starting to look like I can pull it off successfully, so I don't want to blow it using wrong materials.
It's not like this is the ONLY project I have cooking......!