yes , you can sand the base coat BUT you will have to re-coat the area you sanded and blend out your base , plus if you sand with to rough grit paper it will leave sandscratches in the base coat . if you sand base try with a grit of 800 . most times if you try to get dirt out of metalic paint you cause more problems than you started with .
as for spoting in clear that is left to professionals , i have been painting cars for about 15 years and i still have a hard time doing it . sand to a edge and clear the whole panel .
if you rear trying to do a panel without a distinct edge then you sand your blend area with 2000 then clear your panel up to the blend area . then when you are done clearing the panel reduce your clear with a fade-out thinner and then very lightly spray your blend area and a little of the cleared area to burn the 2 areas .
let dry , then buff lightly .
i hope this helps you . maybee if spray tech reads this he will have some helpful tips also .
by the way ,this works with standox ( high line dupont )