It depends on what is under the paint. Some paint is kind of clear and the color of the primer affects the color you see. Some paint is opaque and you just sand it enough to rough it up so the new paint will stick. There's no point in removing the paint and putting the same thing back on. That takes care of scratches.
For rust or peeling paint you need to sand down to the layer that is still okay. Anything peeling must be removed. Most body workers don't know for sure what's under the paint or whether someone else has previously repainted an area so they will often want to sand to bare metal so they know what they're starting with.
They also have to be concerned with what types of paints bond to what types of primers and sealers. When they don't know what those first few layers are, they will usually want to go to bare metal, then build up with multiple layers the stuff designed to work together.
Most newer paint jobs include multiple layers of clear-coat too which is a paint with no pigment. The paint is dull but tough. The clear-coat gives it the shine. When just the clear-coat is scratched of peeling, they will often just lightly sand the paint and feather it into the surrounding clear-cote, then repaint just new clear-coat and blend it in. They will typically paint all the way to the edges of a panel. Those natural breaks hide any slight mismatches that always occur.
Friday, March 16th, 2012 AT 2:59 PM