If you’ve been following Kaser Blasting & Coatings on social media, you know that adhesion has been the theme of 2018.
Good adhesion is the foundation of a high-quality powder coating job. It’s the reason we blast: removing mill scale, rust, and old paint provides a clean surface for coatings to adhere to, and extends the life of the powder coating. Adhesion is the reason we pre-treat: powder won’t adhere to a part that isn’t clean. The better the powder adheres, the more protected the part.
While we always do everything in our power to promote adhesion, some factors are outside of our control. Three to four times a year, for instance, Kaser will see a galvanized part come through the door. Galvanized surfaces can pose some interesting challenges to adhesion and surface smoothness, and since we prioritize customer satisfaction, we like to educate our customers on the risks of powder coating a galvanized part.
Galvanization is the process of coating steel or iron, often by dipping the entire part in molten zinc. It’s very effective in preventing rust; however, it results in a standard gray or silver color, without any options for customization. Lower-quality galvanizing jobs can also result in patchy surface roughness, much to the customer’s dismay.
If a customer consults with Kaser before galvanizing a part, I typically recommend the following: bring us your raw steel part for blasting, a zinc-rich epoxy primer (in powder form), and a super durable polyester top coat (powder coating). Not only will this protect your part against rust, but it will ensure a smooth surface in a variety of color and texture options.
If the part has already been galvanized, our best option is to blast it with crushed glass. This won’t fix any existing surface roughness, but it will impart a bit of texture (called surface profile), giving the powder its best possible chance at adhesion. Crushed glass is used to avoid embedding steel grit in the galvanized finish.
The real trouble comes in the powder coating oven.
When heated, a galvanized part releases gasses in a process known as outgassing. Some galvanized parts produce more of these gasses than others. Meanwhile, the polyester powder starts to crosslink in the oven, forming long, smooth polymer chains. The gas released by the galvanized part can get trapped in the crosslinking powder, creating tiny surface bubbles, and ultimately compromising adhesion. This is tremendously disappointing to a customer who was expecting the smooth surface.
A hammered or textured powder coating finish can help mask some of these imperfections, but unfortunately there isn’t much we can do to stop them from happening.
In summary: if you’re looking to protect your part from rust, powder coating will offer you more color options (and a smoother surface) than galvanizing. If you wish to powder coat your galvanized part, it can be done, but not without risk.
As always, we’re committed to empowering our customers through education. Galvanizing isn’t the last word on rust prevention, and we’d love to discuss other options with you.