Yeah you can sand the base coat down if you'd like, it will help get rid of much of the dirt, but if you do so i'd definetely put down another coat or two of base to cover the scratches you just put in... i'll sand it down too especially if i have to put on many coats to get coverage... the more coats the more dirt.... and I know many yellow's are bad for coverage..... so have him sand it just make sure to put a few coats on afterwards..