Once the base sanding is done, a series of smoothing sanding steps follows. Here, the geometry is no longer to be improved, the surface is simply to be made smoother so that it can be painted at the end.
Recommended in each case are "one and a half steps", e.g. grits 80-120-180-240-320. However, this is not quite accurate. You can, of course, take smaller steps. Again, the pencil trick is helpful here.
The final grain size depends on the planned finish.