100 grit rule
It used to be called the 80 grit rule, until 100 became more available and widespread for long boards..........at least that's how we referred to it......basic 100 was never in the mix.........it was 60, 80, 120, 220, then 400-600 wet.
I still use 80 generally......it's how you final-shape the car with blocks getting ready for the primer coats..........With some high solids over 80, it's nice to block with 100 to expose deeper sand scratches (just fog some black primer over your high solids before you block it.........the black exposes the deeper areas very nicely....takes out the guesswork).
After that, you're ready for some smoother sanding.....(and please use blocks...not a DA (random orbit).......my 2 cents........you can't get a car flat without good blocks). 8)