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.
“Paintlok” (or more precisely, Electro Paintlok) 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 (can be used to enhance paint adhesion). By the way, this stuff is old school — 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.