Method of applying protective coatings to metal products

Claim 1

In the method of providing a metal body with a protective coating that is resistant to chemical and mechanical attack and that comprises successive layers of zinc, chromate and synthetic resin applied to the metal body, in which the zinc is galvanically coated onto the metal body to have a finely crystalline, highly homogenous structure, the improvements comprising 1. applying a chromate layer over the zinc layer by immersing the metal body in a chromic acid solution having a pH value over 2 and containing a reducing agent to form a largely trivalent chromate layer; 2. rinsing and drying the chromate layer to form a layer containing water of crystallization; 3. applying a layer of fluoroplastic synthetic resin dispersed in a liquid over the dried chromate layer; and then 4. thermally treating the metal body to dry the fluoroplastic layer and cause it to interlock with and adhere to the chromate layer and remove said liquid therefrom while preserving water of crystallization in the chromate layer.