Soak In the Benefits of Wet Blasting for Coatings Prep

Achieving high levels of accuracy, consistency and repeatability is critical for every surface finishing process. These characteristics result in efficiency for both the operator and the surface finishing shop — it’s a win/win. Therefore, when choosing between wet (vapor) blasting or dry grit and bead blasting processes as surface preparation for coatings, wet blasting is a popular choice because it leaves surfaces in the optimal condition for coatings adherence. It is also a clean blasting method that enables complete process control, is environmentally friendly and offers reduced media consumption, to name only a few advantages.

Products Finishing talked to one company that develops and produces surface treatment machinery to learn about what exactly wet blasting is, its benefits and when it can be the most advantageous for an application. Vixen Surface Treatments in Thornaby, United Kingdom, not only manufactures wet blasting machines but also makes washing, degreasing and ultrasonic equipment, which are available in the U.S.

Vixen’s AquaBlast wet blasting systems are used across a range of applications and industries to prepare surfaces for coatings using high process control and cleanliness. The company also offers automated equipment that reduces labor and human error. 

Water, media, compressed air … oh, my!

Unlike its dry blasting counterpart, wet blasting uses a mixture of water and abrasive media (commonly aluminum oxide) that Vixen calls “slurry,” which is propelled onto a surface using compressed air to clean, finish or create a

specific surface roughness and can continue to be used for several applications.

“The media is agitated within the sump of the machine and kept in suspension to ensure a consistent and uniform process is achieved,” explains Chris Owen, export sales manager at Vixen. “The slurry is then pumped to the process nozzle where we introduce compressed air.” He adds that although the slurry is pumped at 1.8 bar pressure, that alone is not powerful enough on its own. So the compressed air aids the slurry mixture with velocity and flows over the surface to achieve the process requirements.

According to Vixen, the lubrication and flushing action of water during the blasting process results in a finer finish compared to dry grit and bead blasting methods. Also, a byproduct of the wet blasting process is washing; therefore, in many cases, a precleaning step is not necessary like it is for the dry grit process.

Other factors can be adjusted such as the air pressure depending on the exact finish to be achieved. Air volume, media concentration/density, water temperature, nozzle diameter, nozzle angle, component-to-nozzle range, process speed, automation if required, and component fixture are some other parameters used within Vixen’s wet blasting process.

“In many cases, parts may be visually blotchy and patchy, and may not look uniform, but also mechanically they may not be uniform. So, the blasting process helps to create an even finish and activate the surface on a mechanical level, so it makes the surface very clean, very reactive and gives it an abraded surface so any coatings can take hold the level of the abrasion,” Owen says. “What we’re trying to achieve is a mechanical bonding between the coating and the substrate of the material.”

More on wet vs. dry

When learning about wet blasting, it is difficult not to mention and compare the dry grit blasting process.

First, wet blasting can be controlled to a high accuracy rate unlike dry grit

blasting. In dry grit and bead blasting processes, the abrasive particles are propelled onto the surface without water involvement. Therefore, a 90-degree ricochet occurs upon impact, which can lead to a less controlled and potentially damaging process.

On the other hand, wet blasting provides cushioning, and the altered angle of the media (abrasive particles) in combination with the water’s flow produces a lapping effect. Therefore, slurry travels across the surface of the component being blasted, resulting in a satin/polished

finish that is desired and preferred by many customers.

“With dry blasting, we are trying to control many particles loose in the atmosphere, which is more difficult than when they are suspended within a fluid,” Owen says. “So, the finishes that can be achieved are to a higher consistency, with a more repeatable process, creating a uniform finish.”

Because wet blasting can be controlled to such a high degree, it is sometimes referred to as the gentler process, although that is not always the case, according to Owen. However, the process is gentler on the media and therefore lasts longer within the machine, enabling extended use.

“When dry blasting with aluminum oxide, glass beads or other media, it’s a wildly aggressive application,” explains Jim Jaworski, founder of Founders Equipment and Chemical Sales (Chalfont, Pennsylvania), and Vixen representative in the northeast. “But using the same media in a liquid slurry provides more control of its flow and impact, and ultimately the finish of the part.” 

Media impregnation or embedded media in components can occur with dry blasting methods. However, under normal circumstances, this does not happen with the use of wet blasting because of the gentleness of the process as well as the ability of control to a high degree.

By introducing liquid into the process, wet blasting also eliminates dust. Therefore, the method creates a better, cleaner working environment. Not only is it cleaner for the operator and the space but also for the parts. Owen explains that unlike dry blasting that adds a positive charge to the component, which creates static electricity, wet blasting suppresses any static buildup and gives components a negative charge. As a result, a cleaning process can follow wet blasting and no other particles will be attracted to the parts, providing a cleaner surface for coating.

Wet blasting is also a safer alternative to etching or other chemistry-dependent surface preparations. While some detergents can be used in wet blasting, they are typically very mild. “You can get a clean part that is ready for coating without needing to deal with harsh chemicals or degreasing solvents,” Jaworski says.

Wet blasting generates less heat compared to some dry blasting methods. This is important when dealing with heat-sensitive materials, as it helps prevent thermal damage while achieving the desired surface roughness.

Limited limitations

With all wet blasting’s advantages, it is not easy to find a downside to this method of coatings prep. However, there are a couple potential applications where the use of wet blasting would not be beneficial.

For example, if a finishing shop is working on parts that must emerge in a dry state from the blasting method or if a mirror finish is desirable (wet blasting results in a satin finish), then wet blasting might not be the most appropriate blasting technique.

But even if a component must be dry in the end, Owen argues there are automation options in Vixen’s wet blasting system that apply air to completely dry the component.

Equipment for many industries and needs

Vixen offers a range of wet blasting equipment for an array of industries, whether a shop is looking for new equipment for a specific application or to modify an existing wet blasting model.

The company also offers automation options. “An automated system reduces labor time and provides consistent results,” Jaworski explains. “We customize automation to fit the application. We can fixture the part or rotate it while the nozzle moves around it, changing angles, varying bast times and pressure. We can focus on critical areas of a part or we can blast all the surfaces.”

The company’s automatic Aquablast line includes a radial arm that is also referred to as flat table processing and is often supplied to the aerospace industry for surface prep on engine components. It is also often used by tooling manufacturers for carbide cutting tool surface prep. Indexing machines are also available from the company as well as robotic equipment.

Vixen also provides custom-made multistage wash and blast systems that enable degreasing, washing, wet blasting and then an ultrasonic bath all in one step, Owen says. They are designed to minimize the amount of manual handling during the wash process and are PLC-controlled.

Vixen Surface Treatments | vixenaquablast.com