Sometimes it's hard to get a good technique with bondo. Ideally, you should apply one coat that is exactly the right fill or slightly under ... sand just a bit to get it cleaned up so that it's a good "base" that is about 80-90% right with no high spots (everything the right level or a little low). Then, make a second application that brings that base up to just a little higher than level. This gets sanded to level and you are done.
There might be a case where you have some small bubble holes to fill in but those will be minimal if you mix the bondo by "folding in" with your spreader and not swirling with a stick or screwdriver. Doing that just adds air pockets causing you trouble later. The spreader method keeps air out.
When I started, I'd have WWAAAYYYY too much bondo and sand forever. I was thinking that I'd apply bondo just once so I better get enough on there and shape it with the sandpaper. Wrong. You want to be an expert with the spreader, get it very close with that and sandpaper is just for a little leveling. If you really have bad technique and apply too much, get a cheese grater type bondo tool to get it down fast. That's a bit of a hack method but it'll get it back down to level so you can try again to be more precise. Alternatively, get some really low grit paper (I've even used a grinder with 36 grit because I got so crazy

) and cut it down fast, then back down to something more reasonable to continue getting to level.
Stolen from a website: Sandpaper grits are largely a matter of preference. Here are some basic guidelines:
24 to 36-grit for roughing out the filler with an air file sander or hand board
60 to 80-grit for smoothing out the rough scratches from the previous operation and shaping finer contours
80 to 180-grit for further smoothing in preparation for high-build catalyzed primers (with hardener), or 240 to 320-grit for lacquer-based (non-catalyzed) primers
320 to 600-grit for final sanding prep before paint, following the manufacturer's guidelines on their product information sheet
You'll get dust everywhere from even just a little sanding but if you have a 1" pile of dust on the floor you've applied too much!
Keep working! It'll get there!!