applying filler

Ok give the Fiberglass filler is for strenght and Im going back over it with lightweight body filler. Am I right in thinking that the first layer of fiberglass doesnt have to be completly smoothed out? THen follow with the body filler which needs to be SMOOTH. Whats the best way to initally smooth out body filler? Im guessing some agressive sanding/grinding at first then the closer I get I want to switch to block sanding?