I take it your asking if its alright to sand the base coat because of dirt? this isnt what I call a spot repair if you havent even layed your clear down yet.
If you have a little dirt in the base , just hit the top of the dirt speck with 800 grit, not the whole panel, then re-tac, I wouldnt worry about the minor dirt pieces , because the more you mess around trying to keep dirt out you end up with more dirt , just spray the color , and clear , people worry to much about stuff they have little control over , there is air bound dirt you will never get rid of ! and most dirt comes off YOU, the air hose , the gun , walking around and dragging the hose across the floor stirs up dirt , if you base is that dirty you need to find another area to spray color in .

SprayTech