Epoxy/paint questions

Try 60 first. 40 takes a lot of effort to move. 80 is super slow.

An automotive paint supply should sell self-adheasive rolls of sand paper.

24" skimming knives are tricky to use. I would fair as far as possible before adding another layer.

When I fair, I use one diagonal direction then drop a grit for the other direction. Then drop another grit to return to the first diagonal.

 

For large areas with straight sections I prefer a knock down knife:
ANVIL 24 in. Blue Steel Knock-Down Knife-82132 - The Home Depot https://www.homedepot.com/p/ANVIL-24-in-Blue-Steel-Knock-Down-Knife-82132/308729481