sand from 100 to 150 to 180 or 220.
Or don't sand at all, but use a cabinet scraper....old school.
Read some of these:
http://search.yahoo.com/search?ei=utf-8&fr=ytff1-tyc-inbox&p=sanding%20vs%20scraping%20wood&type=
Depending of the quailty of your planing skills you work may be left with a smooth surface where the wood fibers have been cleanly sheared. Unless you sand in a straight line with a sanding block with the grain direction, sanding marks will show to some extent. Sanding abrades the wood fibers, rather than shearing them off cleanly, so you have to work your way up the grit scale to remove the previous swirl marks if you are using a ROS, random orbit sander.
After the finish is on, and has dried you can sand with a fine grit like 400 or 600 with some water or mineral spirits to get a smooth finish. Buffing with a compound will make it mirror smooth.
If you use a slower drying finish like a poly, you may get dust specks in the dried finish, at least I do. I then use my cabinet scraper to just kiss those nibs off a leaving a very smooth surface.