Powder Coating Bonderized Steel

Q: What, if any, pretreatment is necessary for powder coating over Bonderized steel?

A. First of all, a point of clarification. Many people confuse Bonderized steel with Bonderite, a family of metal pretreatments. Bonderite is a Henkel brand associated with cleaning and pretreating an array of metals and has been a mainstay in the coatings industry since the late 1950s. Bonderized steel, however, has little or nothing to do with Bonderite metal pretreatment. 

Bonderized steel consists of hot dipped galvanized steel (G90 grade) that is pretreated with a phosphate followed by a chromate sealer and dried off in an oven. The process produces a dull, gray-colored finish. Bonderized is commonly referred to as “Paint Grip.” This product is commonly used for roofing, wall panels, and coil. Here is a link for more information: bonderized.com

As for “Paintlok” or more precisely Electro Paintlok, it refers to electro-galvanized steel that has been pretreated with phosphate. The zinc coating is rather thin and withstands most forming operations. It has a dull gray matte appearance and is intended to be painted. Electro Paintlok is described in ASTM A591/A591M Standard Specification for Steel Sheet, Electrolytic Zinc-Coated, for Light Coating Weight Applications, with supplemental phosphatizing specified which can be used to enhance paint adhesion. By the way, this stuff is old school as Electro Paintlok was developed and trademarked by Republic Steel in 1942. The International Steel Group is the most recent owner of the trademark.

Galvannealed steel, which is considered a type of Electro Paintlok (see ASTM A653), is very similar to galvanized steel except it is comprised of a zinc-iron alloy rather than just zinc. Both are described in ASTM A653, with galvannealed designated by A or ZF and galvanized by a G or Z designation.

Both galvannealed and electro-galvanized are readily formable and weldable with no need to remove the coating. Both accept paint well. That said, normal hot-dipped galvanized (ASTM A653 any coating G or Z) can be problematic for paint adhesion.

Speaking of adhesion, Bonderized steel and Electro Paintlok already have a phosphate pretreatment. Nevertheless, I recommend a thorough cleaning of surface “oils and soils” combined with an adequate oven dry-off followed by a preheat to drive off gassing before applying a powder coating.