Q:  What is the difference between a wash primer and a conversion coating? 

A:  A wash primer is an organic coating that is based on a vinyl butyral resin suspended in isopropyl alcohol.  It also contains a corrosion inhibitor, such as zinc chromate.  What makes it unique is that there is phosphoric acid in the coating.  The wash primer is applied using a paint spray gun and the film thickness is usually 0.3-0.5 mils.  The acid etches the metal surface and provides for good adhesion.

A conversion coating is a slightly acidic aqueous solution (water-based) of chemicals.  Iron or zinc phosphates are the most common chemicals in the formulation, although other chemical salts are also added to perform various functions.  The metal is usually immersed in a tank containing the solution.  While immersed the metal dissolves very slightly and the phosphate actually plates out onto the clean metal.  After the phosphating process the chemicals are rinsed off in one or more rinse baths, and in some cases deionized water is used to remove the last remaining traces of contaminants.

The conversion coating acts as an excellent base for paints and at the same time provides excellent corrosion protection.

In most cases a wash primer does not perform as well as a conversion coating.  It is often used when for various legitimate reasons a company doesn't want to invest money for a full conversion process.  Because wash primers have a very high solvent content (approximately 6.5 lbs/gal) it is banned in some states, such as California, where strict smog regulations aim to cut down on solvent emissions.

Most sheet metal fabricators who provide quality products to consumers and customers will generally use a conversion coating.