Degassing cannot always be prevented, as you will understand from this document. Some degassing is therefore permitted and cannot lead to rejection. The standard and the code of practice both indicate that a certain assessment distance should be respected. Usually these craters are not visible at the mentioned distances. Outgassing can largely be prevented by properly preparing your materials, applying the proper handling for the powder coating process and handling your materials properly. These are the steps:
Clean all contamination
Clean and degrease the object you are going to powder coat thoroughly and carefully. Be sure to wear gloves so that your hands do not leave oil and grease on the surface of the object.
Beware when storing galvanized steel
Freshly galvanized steel will directly form zinc oxides in the atmosphere and react with elements in the outside air to form a zinc patina layer. If the materials are improperly stored, long-term moisture exposure can result in what is known as white rust. White rust is a surface contaminant and must be removed. Oxides must also be removed.
Controlling powder coating thickness
Some jobs require a thicker powder coating than others, but the desired coating thickness need not be applied in a single layer. By understanding the materials used in a coating and applying multiple layers, degassing caused by a powder coating that is too thick on the object can be prevented.
Use a primer
Applying a suitable primer layer helps ensure that any gases released during the heating process are processed through the primer.
They can then not bubble up into the color layer of the powder coating itself. Obviously, this is no substitute for thorough cleaning and proper pre-treatment.
Adding additives
The gases that escape in the oven penetrate the still liquid coating layer as the coating gradually cures. Eventually, this causes blistering when the coating is fairly hard and no longer allows the gases to pass through. Certain additives can be used that cause the powder to remain in liquid form longer. This gives the gases more time to escape before the powder hardens. Specialized suppliers typically carry “degassing-friendly” powders.
Degassing the object beforehand
Since degassing is caused by gases released by expansion when the object is heated in the oven, this can be prevented by preheating the object before powder coating. During this “pre-baking,” gas bubbles can escape from the material without powder coating. Usually maintain a temperature 20-30 ºC higher than the required curing temperature (see the powder supplier’s specification for this) and then for double the object’s residence time in the oven. Then do not allow the object to cool completely (approximately to 40-50 ºC ) before starting to apply the powder to avoid condensation of moisture on the surface.
Pretreatment adjustment
Besides the escape of gases due to contaminants, vapor or gas also escapes from the pores of the material to be coated.
So also moisture from the chemical pretreatment of a powder coating plant. For this reason, for degassing-sensitive surfaces, it is sometimes better to opt for abrasive blasting as pretreatment.
Furthermore, it is important to assess the galvanized coating. A rough, dull and gray zinc coating is more susceptible to degassing than a shiny, silvery smooth zinc coating. Sometimes it may be that a gray, dull zinc coating can be seen that is reasonably smooth. If so, the zinc layer may have been post-treated by annealing. That is heating the object again so that the zinc layer acquires other properties. You understand that after this second heating, the chance of degassing is greatly reduced.