the outside of the cars I usually airbrush and keep everything together. just to make sure the color is nice and even. then I apply a quick finish so I will be able to clean of dirt or paint without damaging the color
afterwards I start dismanteling the thing and airbrush the parts that are not invisible in altmode but represent parts from the vehicle... this way these parts look exactly like the real vehicle parts.
I see a lot of people painting these parts by hand but you can always tell its painted by hand.
a good example of this is my
eurotrip longarm dismanteling is only required if there is no way to access whatever it is you need to paint or it its too hard to properly apply a finish
always tape off joints or don't apply too much paint on them, take your time, using color layers will give your repaint a lot more depth then applying just one color, mask your windows, go eazy on the dry brush.
have fun