For clarity and for a steel door, there are generally three types of paint:  primer, finish, and top coats.  A primer is a primary coat that is designed to have another coat of paint (top coat) eventually applied to it.  Many door manufacturers use Gray as the primer color, but primers can have color pigments included to make the paint white, bronze, tan, or any other color.  Most primers have been tested to meet basic performance standards which includes salt spray, humidity, and adhesion tests.  A finish coat is the next level up and is what Diamond applies as a standard paint for it's doors and frames.  Finish coats have higher levels of performance specs to meet, and include an abrasion test that primers do not. Generally speaking, finsh paints can be a single coat, direct to metal paint.  The benefits of a finish coat are much better long term durability as compared to a primer, but can also have a top coat applied without any sanding or additional prep (other than cleaning).  A top coat is a coating typically applied over a primer in order to provide longer term durability.  These are composed of acrylic enamels or polyurethane and can have different gloss levels.  Because of the high cost, this is generally done on a per project basis per the owner's needs - and done on site.  Diamond can provide a top coat paint per request - given larger volume for stock purposes, it can be done at a much reduced cost versus field application.  Contact Diamond for information.