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Jul 9, 2024

The Importance of Strategic Surface Preparation in Metal Finishing

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Surface preparation is a critical step in the metal finishing process greatly impacting the quality, durability, and appearance of the final product...

Whether you are anodizing or applying any type of coating, plating, or other finishing, an ounce of preparation is worth a pound (or more!) of cure.

This surface prep step is essential because it ensures that the surface is free of contaminants, rust, or previous coatings that could interfere with adhesion and affect the finish’s durability. The goal is to create a surface that is clean, smooth, and ready to accept the final coating. There are several different ways to accomplish this, from sandblasting to titanium etching, each tailored to specific requirements and the material at hand. 

At AOTCO, we recognize that the key to any successful metal finishing project is rooted in thorough and deliberate surface preparation. That’s why we are committed to employing meticulous, strategic surface preparation methods that guarantee the highest quality outcomes for every project.

Understanding Surface Contaminants

Types of Surface Contaminants

Any kind of surface contaminant can compromise the quality of the final metal finishing. Understanding these contaminants can help you select the appropriate surface preparation methods.

Oils and Grease

Oils and grease are common contaminants that originate from manufacturing processes or handling. These substances can create a barrier on the metal surface, preventing coatings and finishings from properly adhering. They need to be removed using solvents, detergents, or alkaline cleaning methods to ensure a clean surface.

Chlorides and Acids

Chlorides and acids are corrosive agents that can be present due to environmental exposure or chemical processes. These contaminants can cause under-film corrosion, leading to coating failure. Removing chlorides and acids typically involves washing with water or using neutralizing agents.

Mill Scale and Rust

Mill scale, a layer of iron oxides that can form during hot rolling of steel, and rust from oxidation, are common on untreated steel surfaces. Both must be thoroughly removed to expose bare metal in order for proper adhesion. Abrasive blast cleaning is a common method used to remove mill scale and rust effectively.

Loose Coatings and Corrosion Products

Loose coatings and corrosion products from previous treatments can interfere with the new coating's adhesion. Methods like hand tool cleaning, power tool cleaning, or abrasive blasting can remove these alternative coatings and ensure the new coating bonds properly with the substrate.

Other Foreign Material

Other foreign materials, such as dust and debris, can lead to blistering and other coating defects. These contaminants are typically removed through water jetting, vacuum cleaning, solvent cleaning, and other methods to ensure a pristine surface.

Effects of Surface Contaminants on Finishing Adhesion

Surface contaminants have significant adverse effects on coating adhesion and overall performance.

  • Poor coating adhesion: Contaminants act as barriers that prevent coatings from bonding with the metal surface, leading to weak adhesion.
  • Reduced durability of the coating: Contaminants can cause the coating to degrade more quickly, reducing its lifespan and effectiveness.
  • Increased risk of coating failure: Contaminants can lead to defects such as blistering, peeling, and under-film corrosion, ultimately causing the coating to fail.

Common Types of Surface Preparation

There are several different ways to tackle removing various types of contaminants. The most common techniques can be categorized as mechanical cleaning, chemical cleaning, and blast cleaning. 

Let’s look at the most common methods of surface preparation and the pros/cons of each. 

Blast Cleaning

Blast cleaning, like sandblasting or grit blasting, involves propelling a stream of abrasive particles against a surface under high pressure. This method is highly effective for removing dirt, rust, mill scale, and other contaminants, creating a clean surface suitable for coating adhesion.

  • Process: Abrasive blast cleaning combines air pressure and loose abrasive material, such as sand, steel grit, or aluminum oxide. The abrasive particles impact the metal surface, removing contaminants and cleaning the surface to enhance coating adhesion.
  • Applications: Ideal for cleaning heavily corroded surfaces, removing old coatings, and preparing surfaces for new finishings.
  • Advantages: Provides a high degree of cleanliness and creates an excellent surface profile for coating adhesion.

Mechanical Cleaning

Manually or mechanically removing loose contaminants can be done with hand or power cleaning tools, clearing the metal surface of rust, mill scale, and old coatings.

  • Process: Hand tool cleaning involves using scrapers, wire brushes, or chisels and is suitable for small areas or detailed work. Power tools like rotary wire brushes, sanding discs, needle guns, and grinders provide more efficient and consistent cleaning over larger areas. 
  • Applications: Used for surface preparation where abrasive blasting is impractical or too aggressive.
  • Advantages: Can be used on-site and in confined spaces, offering flexibility in surface preparation.

Chemical Cleaning

Chemical cleaning involves using various chemicals to remove contaminants from the metal surface. 

  • Process: The process depends on the type of chemical being used but typically involves immersing the metal in a solution or cleaning the metal with a solution. Examples include:
      • Acid pickling 
      • Solvent cleaning
      • Detergent cleaning
      • Titanium etching 
  • Applications: It’s particularly effective for eliminating oils, greases, and other organic materials that abrasive blasting might not remove.
  • Advantages: Provides deep and efficient cleaning, removing contaminants that are difficult to eliminate with mechanical methods, leaving the surface ready for subsequent preparation steps.

The Surface Preparation Process: A Step-by-Step Guide

Each step in the surface prep process helps ensure the metal surface is clean, free from contaminants, appropriately textured, and ready to accept the chosen coating material. Not every step is required: it depends on the metal, the contamination, and the final finishing.

1. Cleaning

Cleaning is the initial step in surface preparation. It involves removing contaminants such as dirt, grease, or oil from the metal surface. For light contamination, wiping with a clean cloth or using solvent cleaners can suffice. Heavy contamination may require more aggressive methods like blasting or chemical cleaning.

2. Degreasing

Degreasing focuses specifically on removing oils and greases from the metal surface, common contaminants that can compromise adhesion. Solvents, alkaline cleaners, or acid cleaners that are tailored to the type of metal and the nature of the contamination may be needed.

3. Sanding or Grinding

Sanding or grinding is employed to smooth out imperfections or roughness on the metal surface or to remove particularly tough contaminants. Make sure to choose the appropriate abrasive material based on the metal type and the extent of imperfections.

4. Surface Etching

Surface etching alters the metal surface chemically to create specific textures or patterns. This enhances both decorative and functional aspects of the surface, which is crucial for applications where aesthetics or specific surface characteristics are desired. Titanium etching is one of the most common examples of this process.

5. Coating or Finishing Application

The final step involves applying the chosen coating material or finishing to the prepared metal surface. This may include plating, anodizing, painting, or other processes based on the desired finish and application requirements.

6. Post-Process Treatments 

After the finishing application, certain post-process treatments may be necessary to ensure the longevity and performance of the finished metal product. This can include chem film (conversion coating or chromate), custom masking, engraving, or laser marking. 

Getting Surface Prep Right With the Right Partner

Partnering with AOTCO for your surface preparation needs ensures that you receive high-quality results and a collaborative, efficient experience – at every stage of the metal finishing process.

Whether your project requires basic cleaning or intricate surface preparation, AOTCO is ready to provide reliable, innovative, and effective solutions tailored to your needs. Contact us today to learn more