I normally never sand between basecoats. That is something that we used to do years ago with the old nitrocellulose laquers, but is not really neccessery with the new base/clear systems. I normally spray a coat of base, wait 1/2 hour , spray a second coat of base, then wait one 1/2 hour and spray a third coat, then wait one hour and start right in with the clear. If the base has set for more than 24 hours, then a very light sanding with 600 to 800 is neccessery to make the clear adhere to the base. You can get away with 2 coats of clear, but I always shoot 3 in case I screw something up---that extra coat lets you sand any dirt out of the clear without sanding thru. Since you are going to use a flattening agent in the clear I don't think you should be doing any rubbing out after your final coat of clear.