Don't you mean 2400grit 240 is the stuff you use on wooden doorframes between coats of gloss!! The lowest you'd ever use on a car is 800-1000grit. Any coarser and you'd not be able to remove the sanding lines. By hand you can ensure an even pressure throughout each stroke, which is why its more effective than by DA. On Fiat's soft paint I'd like to bet I could find some lines or holograms left behind by the sanding, compounding or both