Thursday, February 5, 2015

The Importance of Proper Pretreatment in Powder Coating

Why Pretreat for Powder Coatings?
Any given metallic surface has a layer of dirt, oil, mill scale, etc. that must be cleaned and removed prior to powder coating. A coating will not adhere well to a contaminated surface and will result in poor performance, thus making proper pretreatment necessary.

There are a variety of methods that will clean and remove soils from metals. The pretreatment method used is determined by the part to be coated (size, configuration, material), the type of soil to be removed (dust, wax, oil, etc.), and the performance requirements of the finished product.

This process of pretreatment requires three steps:


Mechanical Cleaning – includes methods like scratch brushing and abrasive blasting. These methods remove surface debris such as rust or other types of residual contaminants and scale. This process of cleaning and roughening of part surfaces prior to coating is important and creates a stronger bond between the coating and the part's surface. Some of the most common types of abrasive blasting media include glass bead, ceramic, and aluminum oxide. While mechanical cleaning can be labor intensive, it is frequently the only way to remove excess dirt, rust, or scale.

Chemical cleaning – includes removal of dirt, oil, and grease present on the surface by means of chemicals. The chemicals may be applied by wiping, spraying, or dipping. Application depends on the soils being removed, the size and type of the part, the type of coating to be applied, and the substrate material.

Alone, both mechanical and chemical cleaning will provide good adhesion but will not provide long-term corrosion protection.  For added corrosion protection a conversion coating needs to be applied.

Proper rinsing between process stages removes any remaining contaminated solutions from the substrate surface and is essential for successful pretreatment and coating performance.

Phosphating, or conversion coating, is the application of a phosphate coating to the substrate.  The most common conversion coatings are iron and zinc phosphate which are applied for the following purposes:
  • Provide temporary in-process corrosion protection before application of powder coating.
  • Converts the substrate into a uniform surface, promoting good adhesion of the powder coating.
  • Provide under-paint corrosion inhibitor to improve the life of the powder coating.
The following table compares iron and zinc phosphate processes in more detail.

The end product is the most important factor when deciding between iron and zinc phosphate.  Iron works well for indoor use in non-corrosive environments and has several economic and environmental advantages.  When undercoat protection is needed in a more demanding outdoor, corrosive environment, zinc phosphate is the better option.

Pretreatment is the base for powder coating. The highest quality powder coating will show you excellent results ONLY if the pretreatment is done correctly, maximizing the benefits of powder coating. The results of improper pretreatment include appearance of oily patches, pin holes, rust patches, reduced resistance to weather, and overall poor powder coating performance. 

Have a question about pretreatment?  
Contact our technical help today for advice from an expert, or you can always reference our trouble shooting guide online first.  


  1. Yes, I totally agree with you. Meta pre-treatment is very essential for protecting metal surfaces from corrosion. Keep up the good writing.

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