Questions:
#1 What are you painting?
#2 What condition is it in?
#3 What kind of paint are you using?
#4 What is your skill level with a DA or in line sander?
#5 What's the temperature where you are?

In short there are a number of factors involved in modern painting. You have to almost be a chemist to properly select, mix and apply today's prep and paint. Yes you can block sand with 500 grit wet or dry, but it depends on what you are sanding. Same with a DA if you're applying etch or primer. Modern paints require the matching prep materials or you will waste BIG bucks. If anyone ever wonders why it costs $1500 to paint a car, then they need to go visit a body shop for a day or two.