Oldpinguy, kinda late to your party and your probably already done the filling part , but i thought i would give a couple tips anyway. firstly when i am sanding a cab i try not to take it all the way to bare wood, i try to leave most of the paint as it helps as a guide coat, showing the dents and gouges ect, especially with particle board and that just eats the primer and paint, it keeps the wood fibers sealed.
stay away from kittyhair ( bondo with fiberglass strands) unless your filling large through holes. two part regular bodyfiller is what you want.
MIX THE CAN!!! the can will be sitting on the shelf for who knows how long and the resins separate.
when i am adding the hardener i usually do about a dollop and 3/8 thick and golf ball size . i run one bead across the filler of the hardener ( i know most instructions state to make an X across that always seems way to much and you end up with a half can of filler and no resin hardener. and knead it through with the spreader.
less is more!! i put down enough to fill the scratch and scrape it clean, it's easier to add more later then spend hours sanding it down.
after i fill, and its set up i like to use a rattle can of high fill primer and give it a quick hit. most will not do this but it gives me a guide coat to work with, it seals the wood fibers so i dont oversand and create a new issue beside the repair i am trying to do.
when sanding i use a dual action orbital palm sander, one the lowest setting and let the machine do the work. if your in a hurry and am balls to the walls fastest setting and putting pressure as you sand, it makes too much heat, the filler then melts instead of sands and will plug the paper so fast.
if the sander feels "wobbly" on the surface your working on, check the paper, you might have a build up starting. if you see one, knock it off it you can and continue.
after each fill/sand i take 100watt bulb ( trouble light) and look the panel over in many directions looking for issues. i circle with an HB pencil for filling , and do a zigzag for more sanding. pencil wont bleed through filler and primer like a marker will.
i use 120 to strip , 120 to sand the filler / primer , once filling is where i want it to be i then switch to 220.
i am working on a sinistar cab and will post a few pics later today.