Also remember there is a critical time with Synthetic Enamels--known as re-coatability. If you have JUST painted-it will take another coat--or if it has cured for a good while(like a week or better) it will take a another coat. BUT in between it will lift like you poured paint remover on it.Check your label, or talk to the dealer/manufacturer to find this out for sure.
Most likely if you are sanding it , it is past the critical stage-- but it doesn't hurt to be aware so you don't have a BIG problem.
And yes, as stated above, after sanding off the pimples ,bumps ,runs and orange peel you will have to put another coat of OD or you will have a "shiny" look.
Hope this helps, Jim