What causes 'fish eyes' in powder coating?

Fish eyes in powder coating are defects in the finishing, when there are contaminants present during application.

Although powder coating is a robust and adaptable alternative to liquid paint finishes for a range of applications, there can be issues with finishes. These are usually down to problems with the surface, or the preparation of it.

What Do Fish Eyes in Powder Coating Look Like?

Cratering can occur in powder coating finishes, and fish eyes are a more severe form of craters in the powder-coated surface.

Essentially, a fish-eye is a small depression in the surface with a mound or dome in its centre.

This defect’s resemblance to a fish-eye is how it has got its name.

These defects differ from other dimpled craters in powder coating because they occur for different reasons, and they can appear more severe.

Typically, craters will occur when there are issues with either the surface of the substrate you are spraying, or with the electrostatic charge during application.

Fish eyes come from issues arising from the surface tension differences between the powder film, in molten form, and a contaminant.

Surface tension is the elasticity of liquids, which makes them acquire the least surface area possible. At the interface of a liquid with a surface, there is an uneven force distribution of molecules.

This is why surface tension can be a major factor in several coating defects,  including fish eyes.

What Causes Fish Eyes?

Typical contaminants on a surface that can cause fish eyes are silicone and oil. This contamination may have occurred before any pre-treatment of the surface, prior to applying the powder coating.

However, sometimes these contaminants can occur in the powder itself.

This can depend largely on the quality of the powder you choose. Some powders will cope much better with contaminants than others. High-flow resins are also more susceptible to developing craters and fish eyes.

Unlike craters, which may not reach the surface of the coated substrate, and present a dimpled effect, fish eyes are gross defects, creating deep, circular holes in the powder coating finish.

Fortunately, there are ways of repairing fish eyes and, even better, of preventing them happening in the first place.

How to Prevent Fish Eyes in Powder Coating

Preparing your surface for powder coating is a critical part of the process, and proper preparation can help to ensure you do not get fish eyes in your coated finish.

It is also vital to keep powder coating equipment clean and well-maintained to minimise the potential for contaminants to affect application.

You must thoroughly clean the substrate before applying the powder coating to it. Cleaning and pre-treatment are key steps in ensuring the best possible finish.

The point of pre-treatment is to ensure you prep the surface, so that, down the line, the powder coating you apply stays durable and resilient for longer, and that you avoid any initial defects.

The first important stage is basic cleaning of the substrate. There are various specialist solvents and cleaners for this, depending on the nature of the substrate. 

Once you have completed cleaning, it is important to rinse well to get rid of any loosened particles, dirt or grime.

Tap water is an option for this, but a reverse osmosis or deionised rinse may be more effective.

Following rinsing of the surface, comes conditioning. This is important to prepare the surface to get the results you want from powder coating. 

There are various forms of pre-treatment of substrates, including:

• Chemical pre-treatment

• Chromate conversion

• Zinc phosphate

• Shot blasting.

Following pre-treatment, you must then make sure the surface is thoroughly dry before applying the powder coating.

Compatibility Testing

Before applying powder coating to a surface, you should test it for compatibility with the coating you intend to use.

This is particularly important where you are applying it to a surface where something other than powder has previously coated it.

For example, some aerosol spray paints contain silicone, which, as a contaminant, can cause fish eyes in powder coating.

Other things that can leave contamination on surfaces include adhesive from sandpaper and fibre from grinders.

Maintaining Powder Coating Equipment

This is another critical aspect of preventing fish eyes in powder coating.

Your powder coating equipment should have a good, clean filtered air supply. Check filters regularly and inspect the air lines to ensure the air is dry and clean.

Excessive signs of oil in the oil absorption unit can also cause contamination, so check for this too.

Keep the gun, hose and spray booth clean. Unclean, compressed air in spray equipment can be a cause of defects in finishing.

Can You Repair Fish Eyes?

The short answer is yes, you can, but you will need to re-spray the affected area.

First, you need to buff the defects with some form of abrasive, such as sandpaper. Clean the area thoroughly with a detergent and hot water

Then wipe the it down with a recommended solvent cleaner.

Re-apply the powder coating to the area.

If you want this area to match the performance of the rest of the coated surface, you should powder coat rather than apply liquid paint.

Consistency is Crucial

Creating even, expert powder coating finishes every time requires a systematic, diligent approach to the process.

This requires having excellent process controls, and regular inspection and maintenance of your powder coating equipment.

For more information about our outstanding range of powder coating products, please contact us.