Q: What is the best way to degrease steel and aluminum prior to coating? More specifically, how can this be accomplished when media blasting is not an option due to the “coarse” surface area left. I know media blasting can be fine, but it is still difficult to get a smooth surface.
A: Here is the lowdown on preparing metal for powder coating. First, the durability requirements of the finish must be carefully considered. If this is a basic indoor application that will not see much wear and tear or moisture, heat, etc., then simple solvent cleaning and possibly media blasting with suffice. For anything that will reside outdoors, you need more than that.
Powder chemistry has a significant influence on durability and performance. For instance, epoxies and hybrids adhere well over blasted metal and can provide decent corrosion resistance with a minimum of surface preparation. Essentially all other chemistries require metal cleaning followed by chemical pretreatment. So, if you are using a polyester, acrylic, or polyurethane powder, it is best to clean the metal first with an alkaline solution, rinse well, then apply a chemical pretreatment.
These are typically phosphate based for steel and chromate based for aluminum. Chromate is rather hazardous (actually carcinogenic), so I would strongly recommend that you investigate alternate materials if you need to pre-treat aluminum.
Other (essentially non-toxic) treatment types are based on zirconates, titanates, and silanes. Process control with these is critical, and therefore it is necessary to carefully follow the suppliers’ application parameters.
It is also very important to recognize that although cleaning techniques may be universally acceptable for both steel and aluminum, chemical pretreatments do not translate across these very different metals. Iron phosphate works well on substrates, but it does not prepare aluminum adequately for high-performance powder coatings. You will need chemistry specific to your metal type.