1.2.2 Grinding 

For more aggressive surface preparation, grinding is employed. This technique uses abrasive wheels or discs to remove significant amounts of material, making it ideal for surfaces with heavy rust or substantial imperfections. Grinding is effective at smoothing out rough spots and preparing surfaces that require extensive cleaning or profile modification. It ensures a solid foundation for powder coating by addressing deep contaminants and surface irregularities. 

1.4 Types of Chemical Cleaning Techniques 

Now that you have learned about the various types of chemical cleaners used of chemical cleaning, let’s learn more about the different types of chemical cleaning techniques.  

1.4.1 Hand Wipe 

If you are concerned about how much it costs to powder coat and are looking for an inexpensive pretreatment, then hand wiping might be a good fit in terms of equipment cost. This is assuming that it is suitable for your application. This process involves applying chemical cleaning agents directly onto the surface with a cloth or sponge. This results in precise control and thorough removal of contaminants such as oils, dirt, and residues. Hand wiping can be advantageous for intricate or delicate components that can be damaged by more aggressive cleaning methods, ensuring a smoother finish after the powder coating process. 

1.4.2 Immersion Cleaning 

Chemical cleaning through immersion is used to pretreat parts for advanced powder coating, particularly suited for large, automated operations. This method involves dipping parts in a cleaning solution, which can be either static or agitated. While static immersion systems are simpler, they require a longer time compared to spray cleaning methods. Agitated immersion systems, on the other hand, lead to improved cleaning efficiency due to the physical movement between the solution and the parts. This agitation, achieved through air addition, blade mixing, or basket hoisting, accelerates the chemical reaction between the cleaning agent and contaminants.  

Despite their effectiveness, dip systems need higher concentrations of cleaning chemicals and longer operation times. They can also leave residual chemicals or oils on the parts, which float to the surface of the solution. Therefore, chemical cleaners used in immersion cleaning are specially formulated to emulsify oils rather than allowing them to disperse.  

1.4.3 Hand-Held Spray Wand 

Hand-held spray wands or steam spray systems are great for cleaning parts in batch powder coating operations. These wands help you achieve an elevated level of precision and thoroughness. This method addresses stubborn soils and hard-to-reach areas that automated systems can sometimes miss.  

Spray wands are used within a controlled booth environment that makes them suitable for pretreating larger parts that are otherwise difficult to transport. While effective, this process can become labor-intensive and less efficient for cleaning large quantities of smaller parts before powder coating.  

1.4.4 Ultrasonic Cleaning 

This type of chemical cleaning is a sophisticated technique that enhances the effectiveness of cleaning chemicals through high-frequency sound waves. For ultrasonic cleaning, transducers are placed either inside or outside the cleaning vessel, which generate sound waves that dislodge particles and contaminants from the material's surface. This technique significantly improves cleaning efficiency by removing dirt and contaminants from even the most intricate or complex geometries, including blind holes. 

It is important to note that ultrasonic cleaning is generally less efficient for larger loads, as heavy or large parts can obstruct each other and reduce overall cleaning performance. 

1.4.5 Recirculating Spray Washers 

Recirculating spray washers help in achieving a thorough chemical cleaning of parts before applying your thermosetting powder coating. These machines operate in multiple stages, each equipped with different solutions tailored to specific cleaning requirements. Although this is a more costly and maintenance-intensive cleaning technique, recirculating spray washers offer unmatched efficiency for continuous industrial coating applications. This technique is capable of handling large volumes and can deliver excellent cleaning results. 

1.5 How to Measure Cleanliness

Once you’ve followed all the cleaning steps for your powder coating pretreatment process, determining the results of the process also becomes very important.  

There are several methods that can be used to evaluate the substrate’s cleanliness.  

Out of all such methods, here are some of the most popular approaches: 

1. White Cloth Wipe Test: Wiping the surface with a clean white cloth can help identify residual inorganic or particulate matter like carbon smut. Although not foolproof, this method helps to quickly monitor cleanliness before you transfer your substrate into the powder coating booth.

2. Water-Break-Free Test: Observing if water beads up or sheets off the surface is another practical way to check for organic residues. A “water-break-free” surface means that there are no contaminants / soil present that could affect the powder coating adhesion. 

3. Scotch Tape Test: To evaluate the presence of soils like carbon smut and weld smoke, you can apply a clear adhesive tape to the surface and then examine the particulate matter that gets stuck to the tape. This method is good for providing a relative measure of cleanliness. 

4. Residue Testing: This is a quantitative approach involving solvent extraction to remove residual soils, followed by evaporation, and weighing of the remaining residue. This technique provides precise measurements of surface cleanliness. 

5. Black Light Test: Using a black light to detect fluorescent dyes or oils can reveal residual soils that glow under ultraviolet light. This method is effective if the soil used in testing is known to shine under UV. 

6. Contact Angle Water Drop Test: By measuring the contact angle of a water droplet on the surface, you can get an accurate assessment of cleanliness. This test yields the best results when performed in laboratory settings. 

7. Infrared (IR) Spectroscopy: Analyzing solvent or salt pellets used to clean or assess the surface for residual soils can offer a detailed analysis of the remaining impurities. This is helpful in confirming surface cleanliness. 

All the above methods provide valuable information related to the cleanliness of substrates, ensuring optimal performance of powder coatings. 

 

2.1 Iron Phosphate 

Iron phosphate conversion coating creates a thin, often uneven layer of metal phosphate on an iron oxide base, resulting in a surface that exhibits a gray to blue finish. The formulation of this coating includes phosphate salts, accelerators, or oxidizers, and optionally, a surfactant package.  

The choice and concentration of these components can have a significant impact on the performance and appearance of the final coating. The applications of this coating can vary, from hand wiping to immersion and spray washers, with the spray washer being the most popular method.  

The effectiveness of this conversion coating depends on factors like time, temperature, chemical concentration, and ph. All these factors play a crucial role in achieving the desired coating weight and quality.  

2.2 Zinc Phosphate 

Zinc phosphate is a nonmetallic crystalline coating that adheres chemically to steel, and offers superior performance in adhesion, coating of recessed areas, and corrosion resistance when compared to iron phosphate. This conversion coating consists of phosphoric salts, accelerators, and zinc salts, but lacks a surfactant package.  

It is important to note that zinc phosphate requires a separate cleaning and pre-conditioning stage due to its inability to clean and coat simultaneously. Before applying this coating, an activator - a mild alkaline solution with active titanium salts - is used to prepare the surface. This leads to the formation of a dense coating with optimal adhesion and corrosion resistance.  

Key factors that influence zinc phosphate coatings include concentration, contact time, temperature, and pH, with precise control necessary to achieve consistent, high-quality results. The automobile industry frequently uses this coating for its brilliant corrosion resistance properties. 

 

4.1 Metal  

When you’re discovering how to powder coat metal, it is equally important to learn how to pretreat metal substrates. Metal surfaces can form an oxide layer that impedes adhesion and the final finish. While you can use rust inhibitors like mill oil, it is helpful to know that they provide only temporary protection. Over time, these rust inhibitors degrade and can create compounds that affect the metal substrate’s finish, making it crucial to remove them and ensure the surface is oxide-free before painting. 

Back to overview​​​​​​​