A guide to powder coating pre-treatment options

Before painting or powder coating a metal substrate, it is advisable to consider what preparation is required before powder coating to pre-treat the product. Pre-treatment involves the cleaning and preparation of a surface before applying a finish, with the following aims:

• Removal of dirt, oils and grease
• Corrosion resistance
• Adhesion promotion

The pre-treatment system choice will vary based on several factors but the predominant factors will be the initial state of the substrate and the desired life cycle of the finished product. 

Heavily contaminated, corroded or very dirty surfaces may need mechanical intervention, such as wire brush, shot blasting or sanding, but for this article we will be focusing on Junair’s liquid-based solutions for pre-treatment which include: 

• Dip immersion
• Online spray
• Static spray, single or multi-stage treatment

You can learn more about these pre-treatment options here.

Pre-treatment specification

If your substrate is only lightly contaminated and free from corrosion, and if your product is not destined for a harsh environment, you might want to consider the most basic pre-treatment process, which would be taking a solvent or impregnated wipe to remove any oil from the surface of the material, before beginning your powder coating process. This will assist with initial adhesion of the paint but does not guarantee consistency, longevity of the finish or prevent corrosion.

When producing larger volumes, for better reliability, improved adhesion and corrosion resistance, we would recommend an online spray pre-treatment system. 

The most common pre-treatment choices

By far the most widely used pre-treatment system is a three-stage degreasing phosphate, spray process. Designed for multi-metal use, there is a variety of chemistry available which generally requires a single chemical wash stage followed by two rinse stages. The first stage is heated for improved cleaning, degreasing and surface conversion. The conversion layer, made up of metal phosphates are the result of a chemical reaction with the substrate, leaving behind a modified surface which is proven to increase adhesion of paints and lacquers as well as reducing corrosion. The suitability of the pre-treatment is often determined for each product’s requirement via a salt spray test, whether it be 500, 700, 1,000 hours.

Phosphate systems can be iron phosphate or zirconium phosphate based, depending on the selection criteria. For longer salt spray results, you will need to consider more advanced pre-treatment systems.  

More complex pre-treatment options

5-stage process

If you’re looking for something more advanced or robust for your pre-treatment system, you can implement a five-stage process. For this, you may use a silane-based chemistry, which requires additional stages to separate the cleaning and chemical conversion processes. 

A Deionised (DI) water system is also required for pre and post conversion stages to maintain the quality and longevity of the chemistry. DI rinses are required for a higher grade quality of conversion coating, as minerals such as sodium or potassium ions in the form of salts can occur with towns’ water rinses. 

With a five-stage process, you can expect further increased longevity and corrosion resistance for the finished powder coat. The silane-based conversion coating systems are free of phosphates and hazardous heavy metals, thus ensures simplified waste treatment and easy maintenance as it produces almost no sludge.

7-stage process for architectural aluminium 

Powder coating is now the preferred finish for architectural aluminium features on modern buildings. The European standards for which have been encompassed under the Qualicoat^TM certification scheme. Whilst also including wet painted aluminium, the scheme gives detailed requirements for any coaters looking to pre-treat and powder coat their aluminium components. 

There are several pre-treatment options under Qualicoat, however, the standard generally used is based on a multi-stage process including Alkali Cleaner, Acid etch, non-chrome conversion and various towns’ water and DI rinses.

Qualicoat^TM includes rigid testing regimes which includes among others, adhesion, acetic acid salt spray resistance and accelerated weather testing. There is also the option within the standard of approval for Qualicoat^TM Seaside a more intensive pre-treatment and finishing method. A seaside class is determined as being within 5 kilometres from any coastal or tidal estuary, and is aimed at more aggressive environments.

Consider your material 

The type of pre-treatment process you implement will depend on the material you’re working with and the product’s shape. For example you may be wondering – how do you clean metal before powder coating?

A typical online spray system can be used for a variety of components provided they can be safely hung on the conveyor and are free draining. If factory footprint is an issue then a dip system may be more suitable. One of our engineers will be more than happy to discuss the requirements and the suitability of each system. 

Longer products may benefit from a immersion dip type system due to the reduced footprint compared to a spray system due to the extended drain zones required with a spray tunnel type system. Alternatively, for complex shapes with lower throughput, a manual lance wash may be appropriate.

Choose Junair for your pre-treatment powder coating plant

We hope this guide has been helpful when it comes to understanding what preparation is required before powder coating. Junair can help you build a pre-treatment plant that will suit you and your product, so you have a tailored solution. At Junair, we are happy to provide a free consultation to help guide you to the correct solution.

We can also help your business build powder coating systems so that your process is fully in-house and built to meet the requirements of your sector. 

Get in touch with Junair today to learn more about pre-treatment plants and powder coating systems.

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