Coating selection is an important part of the design process for most metal fabrications. Unless you’re working with a corrosion-resistant alloy, every fabrication needs protection against its environment, which is usually something to enhance its appearance too.
Powder coating and wet painting are two of the most widely used coating processes. Occasionally the two terms will be confused, which causes problems when reviewing a quote request or specification for a metal fabrication. In this blog, we’ll do a “compare and contrast” exercise, so readers can be better informed about the differences.
Why Apply a Coating?
Let’s start by discussing the context in which coatings should be considered. Coatings address two issues with metal fabrications: they protect against damage and corrosion, and they provide a particular appearance.
Not all metals are prone to corrosion and so don’t necessarily need coating. Stainless steel is a prime example, and aluminum, brass and copper are others. These all produce an oxide layer on the surface that protects against corrosion. Non-stainless steel alloys will however corrode and rust, especially if placed in a damp environment. Coating stops moisture from reaching the surface, and so prevents corrosion.
The second reason for coating is to provide a certain look. Color is usually the main requirement, but a coating can also provide varying levels of reflectivity (gloss) and even add texture. Color may be purely for aesthetics, but can also signify a particular function. (Red for danger, green for something medical, and so on.)
Paint and Powder Materials
Paint is a liquid that dries to leave a solid coating. More specifically, it consists of a liquid solvent that carries pigments and resins. The solvent evaporates after application, leaving resins that cross-link and bind together.
Powder coatings are dry granular polymers, either thermoset or thermoplastic material. They are applied dry and fused with heat. Polyester and acrylic resins are two of the most widely used types.
Application Processes for Paint and Powder
Both types of coating material can be applied by spraying and dipping. In fabrication, spraying is most common, and that’s the process we’ll discuss here.
Powder and wet paint spraying processes are very similar. The steps are:
- Mechanical cleaning of the metal substrate: This is to remove any debris sticking to the surface, and can be done by sanding grit or bead blasting.
- Chemical cleaning/activation: For many years this was done with solvents that removed oil and grease. Today though, the main method is aqueous washing, usually by dipping. This raises surface energy (reducing the tendency of water to bead up on the surface), which improves adhesion.
- Electrostatic coating application: The paint or powder is carried through a spray nozzle and onto the metal substrate by compressed air. To ensure good coverage, the coating is given a positive electrical charge, and the substrate is connected in a way that makes it electrically negative. This way, coating particles are attracted to the surface, including regions that could be hard to access. High velocity helps ensure good adhesion at the surface.
Spray processes can be either manual or automatic. Automatic systems, which include robotic systems, are often used in high volume, low variety situations, (such as appliance manufacturing), but in a high mix fabrication shop spraying is usually manual.
- Curing: Wet paint coatings need the solvent to evaporate. This may be done at ambient temperature or can be accelerated in an oven. Powder coating particles are fused together by heating in an oven.
Durability and Protection
Polymer powder coatings are thicker than those formed by wet paint. As a result, they provide more protection against wear and water ingress and so tend to last longer. They are also more flexible than a cured wet paint coating, which helps resist cracking.
When placed outdoors, both paint and powder coatings are attacked by UV light but powder coatings are formulated in a way that gives them more protection.
Finish and Appearance
Wet paint can be mixed to create any color. This could be to achieve a specific Pantone hue or to match with existing structures. Mixing can be done at the point of application if needed, although matching is unlikely to be as accurate as when done in a dedicated facility.
Powder coat materials are available in a very wide range of colors and specific shades can be formulated to order. Ultimately though, the range of shades is not as broad as can be achieved with wet paint.
Powder coating materials can be formulated to produce finishes ranging from flat to glossy and a variety of textures. Wet paint offers more options for texture, and is essential for the highest gloss finishes, as required on cars.
Powder coating does however produce a thicker and more uniform coating. It’s also far less prone to defects like runs.
Environmental Impact
Solvent evaporation gives the wet painting an environmental black mark. These solvents carry volatile organic compounds (VOCs) that can be hazardous to health. This must be addressed with fume extraction systems that can be costly to run and maintain. (Modern paint formulations are much better in terms of VOC emission but still pose challenges.)
In contrast, the absence of VOCs is a definite plus for powder coatings. There’s also less waste as surplus powder can be gathered up and reused. However, polymer curing needs heat, which means powder coating can have slightly higher energy usage.
Cost and Efficiency
Powder coating has higher initial setup costs. These are offset by lower long-term maintenance costs as, unlike wet paint, the coating rarely needs to be stripped and reapplied. Conversely, wet painting is generally a more affordable coating upfront but usually needs more frequent maintenance if corrosion is to be avoided.
Consider the Application
Reviewing where a fabrication will be used, its expected life, and any particular aesthetic requirements should lead you to the best coating choice. In many cases, this will be powder coating, but as noted throughout this blog, there are some situations where wet painting is the better option.
If you need advice or help on coatings for metal fabrication, we will be happy to help. Contact us to arrange a discussion.