It's the weave of the fibreglass cloth.
You see you have to fill the weave.
So you paint on the high build. Then sand it off again. The hollows start to fill up with High build 2 part epoxy.
You could make up your own, but based on the smell (and after one whiff, I pulled out my organic respirator) you have epoxy, fine powder, and a whole lot of nasty solvent to keep it flowing. If you just mix up Epoxy and Filler, it's much harder to work with.
You can use a different colour each time. So it's easy to see when you have sanded enough. And the dark colours tend to have a very dark surface, and be less dark underneath when you sand them back. So you can tell when you have left some hollows.
I think I'm done with this last coat. I'll know after I sand it back.
Then I start painting proper.
I guess the 90% of the work in painting is prep is about right.
Right now the boat looks like a patchwork of white, grey and blue depending on the high and low spots. (Where high and low is measured in small fractions of a mm).
It could probably be made more fair at a larger scale, but hell, it's going to get scratched the first day it sees the water, so I'll go with the 10 feet rule.
If it looks good from 10 feet away, it's fine.
I want to go sailing THIS year.