Preparing Surfaces for Powder Coating

Surface Types and Their Importance

The material of the surface or substrate plays a critical role in the success of the powder coating process. Common substrates include:

• Steel

• Aluminium

• Zinc

• Stainless Steel

• Brass and Copper

Each of these materials requires specific treatment to ensure optimal powder coating adhesion and durability.

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Steel and Its Different Forms

Hot Rolled Steel

Hot rolled steel is produced by rolling steel in its semi-molten form, leaving a mill scale of iron oxides on the surface. This scale, although protective initially, can easily flake off and compromise adhesion.

• Preparation Required: Hot rolled steel must be pickled or shot blasted to remove mill scale before powder coating.

• Alternative: Some steel mills supply pickled and oiled hot rolled steel, which is ready for coating.

Cold Rolled Steel

Cold rolled steel undergoes further processing to improve its dimensional tolerances and strength. It has a smooth, scale-free surface, making it more suitable for powder coating without the need for pre-treatment.

Zintec

Zintec is cold rolled steel coated with a thin layer of zinc, which provides excellent corrosion resistance. It is ideal for projects that require long-term protection from corrosion and can be powder coated without additional pre-treatment.

• Cost-Saving: Areas not requiring coating can rely on the zinc for protection, reducing both material use and coating time.

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Powder Coating Aluminium and Castings

Aluminium

Aluminium is widely used in various industries. However, it has a natural oxide layer that can affect the adhesion of the powder coating. This oxide layer is typically removed by chemical treatment before coating.

Aluminium Castings

Aluminium castings are prone to porosity, which can cause blisters during powder coating. This occurs due to trapped air within the casting, which expands when heated. Proper design and manufacturing of the die can help mitigate these issues.

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Zinc Castings and Their Challenges

Similar to aluminium, zinc castings face the same challenges with porosity due to air trapped during the casting process. Ensuring the die allows for proper flow and cooling is essential to minimise defects during the coating process.

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Powder Coating Stainless Steel

Stainless steel can be successfully powder coated as long as it is free from grease, oil, or waxes. Parts made from stainless steel often require powder coating for aesthetic purposes, such as flues for boilers or stoves. Curing time and temperature must be carefully controlled to ensure proper adhesion and durability.

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Powder Coating Brass and Copper

Brass and copper can also be coated without major issues, though it’s important to ensure surfaces are clean and free from contaminants.

• Colour Discolouration: A common issue is colour bleed in pastel shades when coated on copper. The copper may show through and affect the appearance, which varies based on the alloy.

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Key Takeaways

• Surface preparation is essential to ensuring the best possible powder coating results.

• Hot rolled steel requires extra steps like pickling or shot blasting to remove scale.

• Cold rolled steel and zintec are easier to prepare for powder coating.

• Aluminium and castings require special attention to address issues like porosity and oxide layers.

• Stainless steel, brass, and copper can be coated successfully, provided proper cleaning is done.

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Surface Preparation: Ensuring the Best Finish with Powder Coating

To achieve the best results with powder coating, proper preparation of the substrate is essential. The surface must be clean and free from contaminants to ensure strong adhesion and a durable finish. Here, we’ll explore the key preparation steps to take before applying powder coating.

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Keep It Clean: The Importance of Degreasing

The first and most important stage in preparing metal parts for powder coating is degreasing. Degreasing removes grease, oils, waxes, and other contaminants that may hinder the adhesion of the powder coating. There are several methods for degreasing metal parts, each with its own advantages and limitations:

Vapour Degreasing: The Most Efficient Method

Vapour degreasing involves using a non-flammable chlorinated hydrocarbon solution that is heated to produce hot vapour. The vapour condenses on the cold metal parts, effectively removing oils, greases, and waxes without leaving water-based residues, which can cause corrosion.

• Advantages: Efficient, no drying required, no water-based residues.

• Drawbacks: Requires specialised equipment.

Aqueous Wash Cleaning: A Common Alternative

Aqueous wash cleaning involves using a large machine similar to a dishwasher. Hot water mixed with strong detergents sprays over the parts, cleaning the surfaces. While effective, it has drawbacks, particularly for ferrous metals, as water can become trapped in joints and lead to rusting if not dried properly.

• Advantages: Effective for non-ferrous metals.

• Drawbacks: Risk of trapped moisture in ferrous metals.

Immersion Wash Cleaning: A Thorough Solution

This method involves immersing parts into tanks of hot cleaning solution, followed by rinsing in hot and cold water. The immersion method provides thorough cleaning but, like aqueous washing, risks trapped moisture, especially in ferrous parts.

• Advantages: Thorough cleaning process.

• Drawbacks: Moisture can get trapped, requiring careful drying.

Solvent Wiping: A Basic Approach

In some cases, solvent wiping is used for larger parts that cannot fit into cleaning machines. It involves wiping the parts with a solvent-soaked rag and then drying them with a clean rag. While cost-effective, this method is not recommended as it can leave behind oil residues if not done properly.

• Advantages: Suitable for large parts.

• Drawbacks: Can leave oil residues if not done correctly.

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Pre-Treatment Methods: Phosphating and Chromating

Two other processes that clean and treat the metal surface are phosphating and chromating. These methods not only clean the parts but also enhance the corrosion resistance and improve the adhesion of the powder coating.

Phosphating

Phosphating involves converting a few microns of the metal surface into a crystalline structure that offers excellent corrosion resistance. There are two types of phosphating:

• Iron Phosphating (less effective)

• Zinc Phosphating (preferred for better corrosion resistance)

Parts are either immersed in a solution or processed on a conveyor system with spraying nozzles. However, the risk of water trapping in parts must be managed carefully.

Chromating

Chromating is similar to phosphating but typically used for aluminium and zinc die castings. Like phosphating, it creates a protective crystalline layer that improves the durability and adhesion of the powder coating.

• Advantages: Enhances corrosion resistance and adhesion.

• Drawbacks: Requires careful drying to prevent rust.

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Key Takeaways: Best Practices for Surface Preparation

• Vapour degreasing is the most efficient method for cleaning metal parts before powder coating.

• Aqueous wash and immersion wash cleaning are acceptable but can lead to trapped moisture, especially in ferrous materials.

• Solvent wiping should only be used as a last resort.

• Phosphating and chromating not only clean but also improve the adhesion and corrosion resistance of the substrate.

What ever the substrate you want coating, get in touch for a competitive quote.