Hey Martin,
Maybe you are over looking an important aspect (or trick depending on how you look at it) of a sanding technique? You always want to block sand a larger area then need be. Let me explain a bit...
Cut a part off the sprue, have a nub on an edge. I now use a finger nail emery board and file "with" the edge until flush and smooth. Sure, I might sand a small amount here and there, but it's not noticable.
After filling seams or holes, I take a piece of sandpaper and fold it so it is stiff. I use a small piece say 1/3 the size of the truck of the car model. I keep it wet with my soapy water and hold it "flat" against the part I am sanding. I have some small rubber block and pads (old putty spreaders) that I sometimes use to wrap the sand paper around.
It is also important to use the correct putty or filler. If it is too hard when it dries you will sand more of the surrounding plastic than the putty. In all my years of modeling I have only used my old trusty automotive spot putty.
Good luck, take your time, it is all about prep work.