Sometimes picture vehicles show up at the last minute. In fact, it is not uncommon for a Graphic Artist to have to create graphics for a vehicle that hasn't even been seen, let alone approved.
At that moment when the scramble is on to apply the graphics to the vehicle in time for the shoot, the question of dirt on the vehicle could arise. Often, a last minute vehicle equals a dirty vehicle.
Too often the poor vinyl applicator is being asked to hurry up, and get tempted to just wipe off the areas where the graphics will go. If they only clean off the area where the graphics are being applied, it will show on camera, and you will hate it. If you dont clean the area, and try to apply the graphics over the dirt, they may fall off, and its going to be hard to burnish them without making streaks.
If it happened, the best fix is to wipe or wash the rest of the vehicle in the same manner that the vinyl area was prepped; it will blend it in. A dusting of baking soda/fly ash aging over helps too.
Sometimes the last minute vehicles are old, and have old paint. If the paint looks like it is chipping or flaking, its almost always best to try to apply the graphics over the dirt. Even waxing an old paintjob isn't enough to prevent the vinyl from lifting the old paint. When the paint gets pulled off with the vinyls, it almost always ends up costing the company somewhat of a vehicle repaint.
Sometimes an entire vehicle repaint, because of a few peeled flakes of old paint.