Powder coating is a process that offers beauty and protection for many different types of substrates. From glass and MDF board, to galvanized steel, powder coating has proven unparalleled in performance over the years. However, like all paints, substrate condition and material are perhaps the most important factors in a long lasting coating. Painting over galvanized steel, called a “Duplex System” can be a very effective way to increase the life of steel while also adding a beautiful aesthetic coating. This system protects underneath the paint and helps prevent steel from rusting in areas that paint may have been damaged during installation or handling.
It is excellent to have a relationship with the galvanizer, as the easiest and perhaps most effective painting of hot dipped galvanized steel, is within 48 hours of it being galvanized. Within this time frame the only surface preparation required, if any, is some roughening of the substrate. Since the galvanizing is so fresh, it has not had time to react with environmental factors and consists only of a shiny, smooth, pure zinc layer. By roughening this bright layer, you will give your paint some extra bite. It is very important that the galvanized steel has not been passivated. Talking with the galvanizer to ensure no chromate or passivation pre treatments were used is vital to ensure good adhesion of the paint.
Unfortunately, most of us do not have the luxury of receiving the material 48 hours after its been hot dipped. If you have missed the 48 hour time frame, then, it is best to find material a year after it has been galvanized. This magical year mark has been determined as the right amount of time for the zinc to become “fully weathered” usually giving it a matte grey color. Fully weathered means that the original pure zinc layer has transformed through oxidization into a rougher, denser layer, covering the entire surface, called zinc carbonate. Zinc carbonate provides an amazing profile for the powder coating, as is the reason why fully weathered galvanized steel is a fantastic substrate for powder coating. Please remember that like all substrates to be painted, the surface must be cleaned and dried prior to starting.
If you have received material that does not fall in the 48 hour to 12 month range it is essential that you treat the surface with a cleaner and then profile the part by sweep blasting. These stages are very critical and require the proper chemicals and dwell time. Usually alkaline solutions, ammonia or solvent in combination with the proper media (abrasive aluminum/magnesium silicate with partial size ranging from 200 to 500 microns). It is essential that the abrasive blaster be experienced as the part should be swept and not a full white blast commonly used on steels. The goal is to provide a proper profile and remove the zinc oxide layer rather than the protective zinc coating itself.
Please remember that no matter what stage the material is at, it’s important all substrates be free from dirt, oil, grease and other contaminants before painting. Chemical suppliers can assist you on which detergent, zinc phosphate or other pretreatment option is necessary to ensure a clean prior to painting.
It’s also important to contact your paint supplier to ensure the powders you are using are compatible with zinc as there are paints that perform better and ones that perform worse over this substrate.
FIELD TRICK: Not sure if your steel is fully weathered? Pour some water on it, is it water break free? If so, your steel has formed a layer of zinc carbonate that with proper preparation will be ready for powder coating.