Way back when I was 16, I made a trade with a local bodyshop (it was a one man shop) to help bondo and sand a few cars in exchange for him painting my '68 F250.
He was very clear, over and over and over, how much he HATED to sand, so he stressed how important it was to learn how to apply and form the bondo for the least amount of sanding necessary.
His advice was something this...
Plan your body fill before you start, have a plan.
Get the right tools to smooth and contour
Apply with a wide knife, knock down and smooth with a wider knife
Under filling the first pass is ok, over filling is not ok
Know when to stop working it and let it set up
If you get frustrated with how a spot is working out, go drink a Dr Pepper and then move on to another spot.
