Your right again Dead. Sand scratches that show up a few weeks after painting is caused by the products both on top and underneath the paint shrinking. Say for example somebody finishes their body work in 80 grit..primes the repair area, doesn't leave proper flash times and then it gets prepped for paint. The painter paints the car and it looks great. A time goes by and sand scratches show up in the repaired area. Base coat is made up of pigment and in most cases solvent, (reducer), as I mentioned earlier all clear have a solvent or reducer in them, so again, you have another product with solvent going over top. These solvents go through the base coat, through the prime and right down to the metal and when they hit metal they bounce up and take remaining solvent left in the primer, base coat and clear coat with them...this action is what gives you chemical adhesion and is necessary for paint to stick. However, as it takes the solvent out, the substrate shrinks, as "Deadbodyman" described. When it shrinks, it leaves behind sand scratches. This is another reason why flash times are important as well as finishing body work with a finer paper. When body work is finished in a course paper, the primer fills the sand scratches, but, the primer in the footprint of the sand scratches doesn't "flash" properly and this is how you end up with scratches appearing even months after the car is painted.