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Galvanizing vs. Powder Coating: A Comparative Analysis
June 17, 2024In the world of finishing and protection, powder coating stands apart; known for its durability and vibrant range of colors. But what happens when a surface is already coated? The question arises - can you powder coat over an existing coating? The technicalities, pros and cons, and the common pitfalls to maneuver around are the focus of this blog post.
The Science of Powder Coating
Powder coating is an electrostatic process where a dry powder is applied to a substrate. The basics include proper pre-treatment of the substrate, applying the powder, and baking to create a solid, durable finish.
A quick overview of the process:
Preparation: The item to be coated must be cleaned and pre-treated to ensure the powder sticks. There are different options such as chemical pre-treatment or sandblasting.
Application: This is where the dry powder is electrostatically applied.
Curing: The coated object then goes through a curing oven, melting the powder and forming a hard, durable finish.
Some coating systems include a powder primer and a topcoat.
This simplicity does not account for the complexity that comes when coating over previous layers.
Liquid Paint as Substrate
Coating over a liquid paint is not advised—and here's why. Liquid paints often have unknown chemical compositions, and varying degrees of wear and tear that can be hard to assess, which could jeopardize the new coat's integrity.
Adhesion/Bonding Issues
When questioning whether it's feasible to apply a powder coat over a liquid paint, adhesion becomes a primary concern. The coating may not stick properly to the surface, leading to peeling or flaking. Without knowing the exact type or condition of the old coating, the bond between the powder and the substrate is uncertain at best.
Potential Pitfalls
Even if the liquid paint visibly appears intact and viable for a new coat, there may be underlying deficiencies. The new coat could simply magnify the shortcomings of the old one. Furthermore, liquid coatings are generally not designed to withstand the temperatures that powder coatings are subjected to during curing. While they may appear intact once the product has cooled, the mechanical and aesthetic benefits of powder coating may fail due to the lost integrity of the liquid coating beneath.
Bottom line - we do not recommend powder coating over liquid unless it is part of an engineered coating system.
This base coat was not prepared properly. The top coat easily delaminated.
Powder Coat over Existing Powder Coat
Is it possible, and is it practical, to powder coat over an existing powder coat? The game changes, but the complexities remain.
Assessing the Old Coat
The crucial first step is evaluating the existing powder coat.
- Was it applied correctly?
- What kind of surface prep was originally used?
- Is it compromised in any way?
- How long has it been in use?
- Are there any contaminants that are on or perhaps have penetrated the original coating?
Layering Considerations
Applying a powder coat over another introduces the risk of any pre-existing defects passing through and potentially compounding with the additional layer. The thickness of the existing layer needs to be assessed as well. Applying powder coatings too heavily can lead to a host of issues that compromise not only the appearance but also the performance of the finish.
Excessive thickness can cause the coating to sag or run, resulting in an uneven surface that undermines the aesthetic quality of the finish. Beyond mere appearance, there are functional concerns as well. Thick layers may crack over time once the product is put in use. This cracking not only affects the visual appeal but also diminishes the protective qualities of the coating, making it susceptible to corrosion and degradation. This can increase the likelihood of chipping or peeling, leading to premature failure of the coating system. Furthermore, excessive thickness can impact the flexibility of the coated item, reducing its ability to withstand impact or deformation without the coating cracking or flaking off. Hence, achieving the correct application thickness is critical for ensuring the longevity and effectiveness of powder coatings.
Checking the thickness (DFT) of the existing coating is crucial
Layering Considerations
Applying a powder coat over another introduces the risk of any pre-existing defects passing through and potentially compounding with the additional layer. The thickness of the existing layer needs to be assessed as well. Applying powder coatings too heavily can lead to a host of issues that compromise not only the appearance but also the performance of the finish.
Excessive thickness can cause the coating to sag or run, resulting in an uneven surface that undermines the aesthetic quality of the finish. Beyond mere appearance, there are functional concerns as well. Thick layers may crack over time once the product is put in use. This cracking not only affects the visual appeal but also diminishes the protective qualities of the coating, making it susceptible to corrosion and degradation. This can increase the likelihood of chipping or peeling, leading to premature failure of the coating system. Furthermore, excessive thickness can impact the flexibility of the coated item, reducing its ability to withstand impact or deformation without the coating cracking or flaking off. Hence, achieving the correct application thickness is critical for ensuring the longevity and effectiveness of powder coatings.
This coating failed in the field - it measured over 30mils DFT.
Surface Preparation of Existing Coating
The preparation method can vary from a simple cleaning to a more aggressive approach like sanding, grinding or sandblasting, depending on the age, condition, and requirements of the new coating application. Your powder coater needs to thoroughly assess the existing coat to determine the best way of surface preparation.
Cleaning: The surface must be thoroughly cleaned to remove any contaminants such as oils, dirt, or grease. Common methods include solvent cleaning, alkaline wash, or shot blasting, depending on the nature and severity of the contamination. A clean surface ensures that the powder coating can adhere properly without any interference from residues.
Roughening: For powder coating to adhere effectively, the surface needs to be roughened or textured. This increases the surface area for the coating to bond with. Techniques such as sandblasting or acid etching are employed to achieve this texture. It's essential to control the degree of roughening to match the requirements of the powder coating being applied. A brush or sweep blast can roughen the existing coating to ensure proper bond.
Out-gassing: Prior to the application of the new powder coat, the substrate might need to be preheated if it was previously used or in service. Old materials can accumulate contaminants and moisture, which, if not removed, lead to imperfections in the new coating. During the curing process, trapped gases expand, causing surface defects like bubbles or blisters. Hence, out-gassing improves the chance of a smooth, flawless finish in the application of a new powder coat.
Each of these steps must be tailored to the specific material being coated and the desired finish. Skipping or inadequately performing any step can result in a subpar finish that may not adhere well or last as long as expected. Thus, the thoroughness of the surface preparation process plays a direct role in the overall quality and durability of the powder-coated product.
Inadequate Curing Time
Not allowing enough time for the recoat to cure can impact adhesion and lead to defects in the finish. Proper cure is essential for any coating process. For more information on the importance of properly curing powder, click this link: https://aegisfinishing.com/the-quality-of-the-cure/
Importance of Compatibility
The new coating must be compatible with the existing coating chemistries regardless of paint or powder to ensure there are no undesirable chemical reactions. Such reactions can lead to a range of coating issues. Reduced bond, blistering, changes to the coating's texture, color, or overall performance, potentially weakening its protective qualities or altering its appearance can be results of chemical incompatibility. It's crucial to thoroughly evaluate the compatibility of coatings to prevent these types of disruptions and ensure the coating's integrity and functionality are maintained.
The top layer of powder has wrinkled overtop of the first coating.
Warranty
It is important to note that re-coating over an existing coating often falls outside warranty coverage, primarily because there is no control over the condition or quality of the existing finish, or the surface preparation beneath it.
Coating warranties are typically based on stringent application and performance criteria which are difficult to ensure when applying new coatings over old ones. The inability to fully assess and guarantee the existing layer's adherence, integrity, and compatibility means that recoating introduces variables that could undermine the performance of the new coating. This consideration is crucial when planning a recoating project, as it emphasizes the importance of thorough preparation and the potential need for warranty discussions with coating suppliers.
Best Practices for successful recoating
- Conduct a Thorough Assessment: Evaluate the existing coating, as well as product age and use.
- Select the Right Coating System: Consider the entire system – primer, topcoat, and any additional layers – to ensure compatibility and performance. If a long term coating is important, we strongly recommend stripping down to the base substrate.
- Prepare the Surface Appropriately: Spend the necessary time and effort on proper cleaning and preparation to create an ideal surface for the new coating to adhere to.
- Apply the New Coat Correctly: Follow the manufacturer's recommendations for application, thickness, and cure to achieve the best adhesion and finish quality.
- Monitor and Maintain the Coated Surface: Regular inspections and maintenance help extend the life of the recoated surface.
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