Blasting vs. Washing

Q: We have a small, batch powder coating system. We coat mostly steel parts that are large welded structures. We prepare the parts with a manual grit blasting process using a medium-sized aluminum oxide media. We have been happy with the results for the most part, but some customers and suppliers have suggested we consider a wash system for better corrosion resistance. We do get some projects that are used outdoors, so corrosion is an issue at times. We have been using a primer coat on this work to get the thickness up and make sure we have complete coverage. We do not know much about different wash processes, so we are not clear on what could be done and what the benefit would be for outdoor parts. Can you give us an idea of the options and what might work best for our operation? 

A: Blasting is a very common method of part preparation for powder coating. What you refer to as “washing” will typically involve cleaning with a water-based cleaner and application of a conversion coating to neutralize the reactive metal surface and possibly provide some corrosion protection.

To understand the options and relative benefits, it is important to know that powder coating is actually a process with different methods to reach a give performance level. The steps to get the result may vary as long as the combination is right for the product performance required.

If you went to a wash process, you would most likely stay with a manual batch process. You would use an enclosure to spray different products onto the surface that could include a number of steps including cleaning, rinsing and treatment of the metal surface. This would typically be a minimum of three steps but could be as many as 7 or more to get outdoor performance. A critical limitation for a batch system is inorganic soils like rust or mill scale. An acid type cleaner or descaler could be used but the strong products may be a problem from a health and environmental perspective. In addition, you would need a good conversion coating like zinc phosphate with a chrome rinse to get a high level of corrosion resistance. Iron phosphate would be a lot easier to use but would not give you much corrosion protection. This wash process would be more labor intensive and require some harsh chemicals so it is not a simple decision. 

Blasting will get the inorganic soils just fine but sometimes is less effective at removal of organic oils and greases. To make sure you get the oils, you need to keep your media clean. Blasting does not add any corrosion resistance so you have to be certain you have a robust powder coating film that completely covers the peaks of the blast profile. Your use of a prime coat adds a lot of thickness and also should give you great resistance to moisture penetration.

A cleaning process in a batch system can be effective if you have the right steps. It works well for cold-rolled steel and other substrates that do not have inorganic soils. You may still need primer for outdoor products. I would stick with the blast process for hot-rolled steel and count on the primer coat to add the needed corrosion protection.

About the Author

Rodger Talbert

Rodger has more than 30 years of experience in the powder coating industry.