Phosphate coatings on steel (also called Parkerizing or Bonderizing) are produced by brushing or spraying, onto a clean surface of steel, a cold or hot dilute manganese or zinc acid orthophosphate solution (e.g., ZnH₂PO₄ plus H₃PO₄). The ensuing reaction produces a network of porous metal phosphate crystals firmly bonded to the steel surface. Accelerators are sometimes added to the phosphating solution (e.g., Cu²⁺, ClO₃⁻, or NO₃⁻) to speed the reaction.

Phosphate coatings do not provide appreciable corrosion protection in themselves. They are useful mainly as a base for paints, ensuring good adherence of paint to steel and decreasing the tendency for corrosion to undercut the paint film at scratches or other defects. Sometimes, phosphate coatings are impregnated with oils or waxes, which may provide some protection against rusting, especially if they contain corrosion inhibitors.

In corrosion-resistant automobile bodies, the first, and probably the most important, layer of the paint system is the phosphate coating. Although it is the thinnest coating in the paint system (about 3μm (0.1 mil) thick), it is the anchor for the subsequent layers.