Why is paint clearance important?

Paint clearance is the normal gap left between the knuckle and the barrel of the hinge when it is made. It's used as a guideline for the thickness of the painting or plating applied to the hinge surface.  Most professional finishers will ensure that they are within the guideline, but if they don't, the error is usually quite obvious.  See here for more information: paint clearance | Anatomy of Hinges | Glossary (guden.com)