Deciding whether you can coat over an older powder coating is an important consideration for many maintenance and refurbishment projects. The ability to apply a new layer of powder coating over an existing one can save time and costs compared to stripping and recoating from scratch. However, several factors influence whether this approach is feasible and effective:
- Significance of Determining Feasibility: Understanding if and how you can coat over old powder coating is crucial for ensuring the longevity and quality of the new finish. Factors such as the condition of the old coating, compatibility of new powders, and proper surface preparation all play roles in determining success.
- Overview of What Will Be Covered:
- Evaluating the condition of the existing coating.
- Preparing the surface for re-coating.
- Application techniques.
Evaluating the Condition of the Existing Coating
Before applying a new layer of powder coating over an existing one, it’s essential to assess the condition of the old coating to ensure a successful and durable finish. In general, the longer it has been since the original coating was applied, the less likely it will be to achieve good results with a new coating. Here’s how to evaluate the existing coating:
Quality of original prep work:
- If possible, verify the preparation process of the metal substrate that was done before the original coating was applied. If you are unable to verify what prep work has been done, or if the prepping process does not meet your standards, it may be best to remove the existing coating. Your new coating is only as good as what is beneath it. If the existing coating fails, so will the new coating.
Signs of Wear and Damage:
- Peeling: Check if the old coating is lifting or separating from the substrate. Peeling indicates poor adhesion, which can compromise the new coating’s adherence.
- Chipping: Look for areas where the coating has chipped away. Chipping exposes the substrate and can affect the integrity of the new coating layer.
- Corrosion: Inspect for rust or corrosion beneath the old coating. Corrosion can spread and degrade the new coating if not properly addressed. If corrosion or rust is present, it is better to strip the part entirely and remove the rust altogether.
Compatibility of Powder Coatings
Ensuring that the new powder coating is compatible with the old coating is necessary for achieving a high-quality finish. Here’s how to assess and ensure compatibility:
- Importance of Chemical Compatibility:
- Chemistry Interaction: Different types of powder coatings may interact chemically, affecting the adhesion and performance of the new layer. For instance, applying a new polyester-based powder over an old epoxy coating without proper preparation can lead to issues like peeling or poor adhesion. It’s crucial to remember that applying a powder coating without UV stability as a top coat won’t provide protection to the finish, even if the base material is UV stable.
- Uniformity and Durability: Incompatible coatings may result in a finish that is uneven or less durable. Proper compatibility ensures that the new powder coating bonds well and provides the desired appearance and protection.
- Consulting with Manufacturers: Reach out to the manufacturers of both the old and new powders to get recommendations on compatibility. Manufacturers can provide valuable information on which products can be used together and any special preparation requirements. Each powder coating product has a Technical Data Sheet (TDS) that provides chemistry information, helping verify compatibility for specific applications.
By carefully evaluating the condition of the existing coating and ensuring compatibility with the new powder, you can make informed decisions about the feasibility of coating over an old powder coating. Proper assessment and preparation are key to achieving a successful and durable finish.
Preparation for Re-Coating
Proper preparation is critical when applying a new powder coating over an existing one. Ensuring the surface is clean and properly abraded can significantly impact the final result. Here’s a detailed guide on how to prepare for re-coating:
Surface Preparation
- Cleaning:
- Remove Contaminants: Start by thoroughly cleaning the surface to remove any contaminants such as dirt, oil, grease, or other residues. Contaminants can interfere with the adhesion of the new coating.
- Detergents and Water: For general cleaning, use mild detergents and water. Scrub the surface with brushes or cloths to ensure all dirt and debris are removed. It is best to avoid solvents such as MEK or acetone as they do not fully clean the surface but rather “push contaminants around” on the substrate.
- High-Pressure Washing: For larger surfaces, a high-pressure washer can be effective in removing loose contaminants and residues. Ensure the surface is thoroughly dried afterward.
- Creating a Surface Profile: To improve adhesion, it’s essential to create a rough surface profile that helps the new coating bond better with the old one. This is usually achieved through sanding or abrasive blasting.
- Sanding: Use sandpaper or a sanding machine to sand the surface. Select an appropriate grit level based on the condition of the old coating—typically, medium to fine grit (e.g., 120-220) is used.
- Abrasive Blasting: For tougher coatings or larger areas, abrasive blasting (sandblasting) can be employed to achieve a consistent surface profile. Ensure the blasting media and pressure are appropriate for the substrate and old coating.
- Surface Condition: After cleaning and abrading, inspect the surface for any remaining imperfections or damage. Look for areas that may need additional attention, such as deep scratches or uneven patches.
- Surface Profile: Check that the surface profile created is suitable for good adhesion. The surface should feel slightly rough or textured to the touch.
Application Techniques
- Spraying:
- Gun Settings: Adjust the gun settings, such as air pressure and powder flow rate, to achieve optimal coverage. Refer to the manufacturer’s recommendations for your specific equipment. For instance, coating over an preexisting coating generally requires the coater to reduce the KV or select the “Second Coat” option (if available on your powder coating system).
- Technique: Hold the gun at the recommended distance from the substrate (typically 6-12 inches) and move it in a consistent, sweeping motion to ensure uniform coverage.
- Coverage:
- Uniform Application: First, apply the powder in thin, even layers at the center of the substrate to avoid unnecessary buildup. Next, spray wide, overlapping passes to achieve a consistent coat, which is known as feathering.
- Inspection: After spraying, inspect the coated surface to check for even coverage and make adjustments as needed.
- Edge and Corner Coverage: Pay special attention to edges and corners, which can be challenging to coat evenly. Using specific techniques such as feathering helps ensure that the substrate is covered properly.