After the filler is applied I start BLOCKING the filler using 80 grit on a long board. I use strips of 80 grit either 17 1/2 inch X 2 3/4 inch for clip on boards or 16 1/2 inch X 2 3/4 inch for boards that use a PSA (Pressure Sensitive Adhesive or sticky paper) backing. Some people use air boards, I personally don't, I feel I have more control if I use a long board that is hand operated. Another reason I prefer using a hand operated long board is the shear weight of the air board because the trick to getting a panel straight is to not apply pressure to the tool your using. By not applying pressure to the long board, the sandpaper does the work and the panel has no choice but to be straight. The other tool I most commonly use is the short block. If you take a 9 X 11 sheet of sand paper and fold it in half with wise and fold it in half again, you can get 4 pieces of sand paper from a single sheet of 9 X 11 sandpaper.
When your blocking filler never increase your grit size by more than 100 grit. So if you start by using 80, your next grit would be 180, many people feel that 180 grit is fine enough for primer and in most case it is. For the added insurance I step up to 240 grit and finish my body work in 320 grit. This, is for all intense and purposes over kill. The reason I finish my body work that fine is that I have virtually no chance of sand scratches coming through 6 months after the car is painted so it's just a bit of insurance I use, it takes just a few minutes more and in many cases it means I can get away with priming a panel with one prime session.
As you noticed, in my first paragraph, I put the word blocking in caps. The reason I did that was because when it comes to sanding filler I don't use any machine at all...I use blocks. You can use a DA, air board or palm sander but if you want the panel to be straight, eventually you will use a long board and a short block. This is my preference, air tools work, I prefer hand tools, I like to get my work straight from the beginning.
Another tip when your blocking filler is to cut your filler with your long board or short block at a 45 degree angle for about 10 strokes, then change to the opposite 45 degree angle for another 10 strokes. That way every 10 strokes your cutting the filler against the grain and it cuts faster. I also use guide coat on my filler, it quickly tells me where my highs spots and low spots are. When you reach bare metal it's time to evaluate if the area that the metal is showing is high or if the rest of the panel is low, run your hand across the panel, don't look at it, look away and let the feel of your hand tell you if it's high or low. If the the area with the metal showing is high, gently tap it down with a body hammer and apply filler and start blocking again. if the area where the metal is showing is where it should be, more filler needs to be applied and again, start blocking.
If you need more clarification, let me know and I'll try and explain further.