Causes and Fixes for Liquid Paint Defects

Q: Could outgassing be a cause of pinholes, rupture or craters?

A: Whether you are a job shop or an OEM involved in liquid paint application, paint defects are something that everyone will encounter at some time. Quickly identifying and eliminating these issues can improve your bottom line by decreasing costly rework and improving the quality of the finished part. A few common paint defects are craters, ruptures and pinholing.

Pinholing appears as pinhole-sized voids on the surface of painted parts, ruptures are bumps or bubbles, and craters are bowl-shaped depressions that penetrate to the surface of the substrate. All three types of defects can look very similar, and outgassing does typically produce these types of defects.

Outgassing takes place when gas is generated under the coating film and migrates out of the coating during the curing process. Outgassing typically occurs when contaminants or moisture become entrapped under the coating film. This occurrence is more prevalent on workpieces that have porous surfaces or voids, or when the substrate has been roughened or etched using mechanical or chemical cleaning methods.

Outgassing can be difficult to troubleshoot and eliminate. The resultant paint defects can appear in random spots, on a specific part or a specific substrate. The high temperatures of the paint curing process can volatize contaminants under the coating film, causing a gas bubble to migrate to and burst through the surface of the coating, and creating a cavity in the coating that may penetrate to the base metal. Since the appearance of the defect can be very sporadic, it can be difficult to determine if outgassing is its actual cause.  

One way to resolve the issue, is to expose the prefinished part to the curing process, baking the metal prior to regular pretreatment and paint applications. Hot temperatures will condition the part, allow contaminants to surface and eliminating the entrapped gas from the substrate prior to the subsequent operations. With galvanized or casted parts, outgassing can also be eliminated by implementing changes to the metal processing or casting process.

Pinholing and ruptures in the liquid spray application can be caused by organic or inorganic soils trapped under applied paint. The sources of the contamination can be a at any stage of the process: shop dirt, paint ash from burnt-off racks, poorly cleaned and rinsed parts, oven debris, poor air circulation in the plant or oil contamination in the airlines used to blow off excess solution as parts exit the washer. All of this contamination can become visible in the finished materials after paint is applied. 

During the troubleshooting process, one must eliminate process contamination as a root cause of the defect, starting with verification of the pretreatment process. Robust pretreatment processes must encompass cleaning, rinsing and the application of a conversion coating. Organic and inorganic soils must be removed to provide a clean base for the conversion coating, ensuring excellent paint adhesion and adequate corrosion protection. All of the suspended solids, such as paint ash or metal fines from grinding operations, free-floating and emulsified oils, must be filtered out or removed. They must not be redeposited on the surface of clean or pretreated parts. Freshwater exit halos have proven to be very successful in rinsing off any dirt from the substrate surface prior to painting, as this allows the parts to be rinsed with the purest water.

It is of the utmost importance to prevent cross-contamination in the pretreatment process, and it is crucial in ecoat applications. Overly rapid deposition of ecoat paint can also result in paint failure, showing up as volcano-type ruptures. The most severe defects are visible on zinc-iron-coated substrates. Galvanized parts also are prone to deformity, while cold-rolled steel almost never exhibits failures in this type of application. If ruptures on the part are concentrated in one area of the rack, close to the anodes, voltage reduction, anode rotation and an increase in the zone gap can help with eliminating this defect. 

One important question to ask yourself when paint defects occur is “What has changed in my process?” Proper documentation is key. Alloy composition of the substrate, supplier information and process parameters should be on file and easily accessible. Any changes to manufacturing, pretreatment or paint process procedures should be updated at the time of a change. This will help with troubleshooting by establishing a timeline for when defects start to occur and reviewing changes implemented in the process.

Preventive maintenance programs also can prove to be helpful in minimizing paint defects and ensuring high-quality finished goods. Frequently scheduled clean out of ovens, air lines, process tanks and equipment minimizes the amount of contaminates in the manufacturing process, resulting in a minimal amount of dirt or soil being redeposited on the prepainted part. 

Process controls also can enhance the performance of a system. Analyzers will provide consistency by taking control of the chemical additions and maintaining good-quality rinses.  Data acquisition systems forewarn of any problems when they are used with auxiliary alarms and are excellent tools in the troubleshooting process, providing historical and trending data.

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Tomasz Slezak is a market manager for surface treatment with Chemetall. Visit chemetallna.com.