The Importance of Pretreatment
Pretreatment is the term applied to the process of preparing a surface to receive powder coat. Powder can be sprayed over bare metal with no special pretreatment and initially look beautiful, and if you are paying $100 to powder coat your Harley frame this is probably what you are getting. Without proper pretreatment, however, the powder will chip and corrode easily.
Pretreatment involves the stripping of any existing corrosion or coating, the cleaning and degreasing of the bare metal, and the creation of a surface finish adequate for a powder coat. The objective is to leave a clean and slightly rough surface profile as an anchor pattern for the powder to adhere to. Some people refer to this profile as “tooth” because it effectively bites into the powder coat to mechanically interlock the coating and substrate.
The ideal anchor pattern to powder coat steel is created either mechanically (typically by abrasive blasting), or chemically (historically with a conversion coating). Abrasive blasting is the best way to remove old paint or rust, and is the only way to remove mill scale from hot rolled steel. We consider abrasive blasting the only appropriate pretreatment for welded steel fabrications. Abrasive blasting is, however, labor intensive and expensive, and may not be well-suited for delicate parts.
Chemical conversion coatings are easily automated and inexpensive, but are environmentally harsh and difficult to do correctly. For aluminum, chromate conversion coatings offer excellent pretreatment. For steel, zinc phosphate conversion coatings are the best. Unfortunately, both these chemical processes involve heavy metals that limit responsible use of them to metal plating shops with sophisticated waste treatment facilities and federal discharge monitoring. Iron phosphate conversion coatings are more popular than zinc for steel parts due to less severe (although still federally monitored) discharge requirements, but powder manufacturers advise that iron phosphate by itself isn't recommended for exterior applications of steel. Finally, all conversion coating pretreatments are only as good as the maintenance of the chemical and the effectivity of the applicator. If the chemicals are not held within a narrow range of concentration and pH (typically requiring adjustment 1-3 times a day through a process called titration), the resulting pretreatment can be worse than no pretreatment. Similarly, if the chemical application is not done correctly the results can be worse (pre-rusting, acid preciptates) than no pretreatment. Manual wand iron phosphate pretreatment is so prone to problems that powder manufacturers often won't extend warranties to shops using only that method.
Pretreatment is very important, complex, and difficult to evaluate after powder coat is applied. Where does that leave those looking for a powder coat service provider? Asking questions, looking at online reviews, talking with references, and researching the subject can provide you with background information that will help you formulate an opinion about the capabilities of a prospective coater. Ultimately you should visit the shop. We, at Extreme Finishes, take pride in keeping our shop clean and organized, and we take the same care with our customers parts as we do with our own equipment. We have the equipment, permits, and process knowledge to properly pretreat virtually any coatable metal, and we work hard to maintain our reputation for high quality, durable, and long lasting powder coatings!