From time to time we receive enquiries about powder coating over a galvanized surface. You can understand why, as galvanizing is still the most effective steel protection method out there, but it doesn't look very pleasing to the eye in some applications, such as on a vehicle.
Applying a durable powder coat, or painted finish for that matter, over galvanizing is not easily achieved for the following reasons.
1. Galvanizing leaves a shiny surface which makes it difficult for other coatings to stick to it. This can be alleviated slightly by using an etch primer, but it is not 100% successful and the powder coat can still easily chip off on impact.
2. The other reason, is that a galvanized surface usually has random lumps and bumps on it, as it is primarily an industrial protective coating rather than a decorative finish. When you apply a coating over the top of galvanizing it looks like a very poor powder coat finish.
One way to enable powder coat to stick to galvanizing successfully is to leave it to weather for a few months which dulls the surface and helps it to stick. But clearly this is not a realistic option for manufacturers.
Some manufacturers try using a zinc-plated surface under powder coating, but this can be more problematic than galvanizing, as although it is a smoother finish, it's even shinier than galvanizing, and is more difficult in terms of adhesion for a surface coating such as powder or paint.
The system we employ is a 4-stage powder process. Firstly, the item is shot-blasted, this removes surface contaminants and creates a rough surface for the coating to stick to. The next stage is immersion in a zinc-rich liquid primer bath. This ensures the whole surface is covered. The thin nature of the liquid ensures all the corners and crevices are coated. The third stage is another zinc coating, but this time the item is heated to 200 degrees and the coating is a powder rather than liquid. This coating is then baked for 30 minutes. So now there are two layers of zinc coating. The fourth and final coating is the black powder. Again the item is heated to 200 degrees and the powder is applied before being baked once again. The black powder is a polyester material. This provides very good protection characteristics, as it is not too hard which can lead to being easily chipped, but it is tough enough to absorb light impact without damage or chipping.
For really tough working environments it can be sometimes preferable to choose galvanizing, as it is still the most effective protection process against steel corrosion there is. But it is not intended to look attractive. When it comes to choosing between durability and appearance it is often down to personal preference. We offer many of our Shadow products in either powder coated or galvanized finishes, to suit all applications.