Powder Coating Q&A: Pretreatment Methods

Are there simple pretreatment options that will work with powder coating and help to limit rust on steel surfaces with a lot of welds and other complex issues?


Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

Q. We are just learning the powder coating business. We have been told that we must get the part clean before we powder coat it, and we are learning about our options. We are also looking at chemical treatments to provide better adhesion and rust protection. Are you aware of any simple pretreatment options that will work with powder coating and help to limit rust on steel surfaces with a lot of welds and other complex issues?

A. A good alkaline cleaner is usually a good place to start with cold rolled steel that is relatively clean in the raw form. It does a good job of breaking down organic rust inhibitors and similar soils. Once a steel surface has been cleaned it is desirable to add a conversion coating to protect from oxidation and to enhance bonding. Iron phosphate works well with powder, but has modest corrosion resistance if the coating is film broken. Zirconium oxide products can also enhance bonding and they provide somewhat better corrosion resistance on different metals. Some of the zirconium oxide products contain other metals such as copper or titanium and are somewhat good corrosion inhibitors. Zinc phosphate has very good corrosion resistance but it also requires waste treatment and would not be considered simple or people friendly. If you are doing welding or using hot rolled steel you should consider an abrasive step to remove inorganic soils such a rust and scale. If you cut steel with a laser cutter you should definitely have a blast or grind operation to take the scale off the cut edge.

One of the best options for corrosion resistance is the use of a powder primer. Thoroughly clean the surface by chemical or mechanical cleaning before adding the primer. You can add a conversion coating or not, depending on the final level of quality you need. The powder primer can contain zinc or not. The zinc-rich primer works very well if the surface is roughened by blasting. It is less effective if the surface is not blasted because the zinc will not develop the kind of cathodic contact needed to provide the sacrificial barrier as intended. With or without zinc, the powder primer will provide exceptional resistance to moisture penetration and a thicker film with better edge coverage. The primer does a great job of stopping the spread of rust.

Originally published in the January 2016 issue.


  • Developments and Trends in Powder Coating

    New solutions for powder coating centers combine powder preparation, conveyance, dosing and color changes into a fully automated, closed system.

  • Calculating the Cost of Powder Coating

    How can you calculate the cost of powder coating a component if you only know its surface area? Powder coating expert Rodger Talbert has the answer.

  • Masking for Surface Finishing

    Masking is employed in most any metal finishing operation where only a specifically defined area of the surface of a part must be exposed to a process. Conversely, masking may be employed on a surface where treatment is either not required or must be avoided. This article covers the many aspects of masking for metal finishing, including applications, methods and the various types of masking employed.