With body work, some of the simplest things can make the biggest difference. One of these is the use of 40 grit. While yes, it does have an important use, it should not be over used. I have been guilty of this in the past, but with always trying to evolve and improve, I have changed how I use 40 grit.
When sanding body filler, 40 grit should definitely be used, before the filler is completely dry, to rough in the general shape that is sought after. It will not only obtain the general correct shape much faster, it will produce a better result because it is so sharp. The important part is knowing when to stop with it.
Once the bulk of the body filler is knocked down, and a very rough shape is there, I stop using 40 grit. Normally, by the time the filler is sanded to this point, it is dry enough to use 80 grit.
80 grit should be used to do a lot of the block sanding. It is more precise than 40 grit, so it can get the body filler dialed in much closer than 40 grit can. You won't be chasing deep 40 grit scratches. By the time a project is done with all the block sanding of the body filler, it will already be in 80 grit, and ready to sand with 150 before primer.
•••Here are a few other thoughts. Filler should not be spread over 40 grit. It is too aggressive, causes too many pin holes, and too much of a chance of those deep scratches showing back up. There will be less glaze used. The quality of work will improve quite a bit. It will be much easier to get all of the filler in 150 grit before primer.