Preparing A Surface for Powder Coating

If you are considering having something powder coated, or are powder coating something yourself, proper surface preparation is a must if the the coating you are using is to properly bond to the surface. Items or parts that are powder coated without being treated and properly prepared will peel, chip, flake and generally NOT bond to the surface you are painting. Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.

There are chemical pre-treatments that involve the use of phosphates or chromates in submersion or spray application. These usually occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal. Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the environment. Titanium zirconium and silanes offer similar performance against corrosion and adhesion of the powder.

Another method of preparing the surface prior to coating is known as abrasive blasting or sandblasting and shot blasting. Different powder coating applications can require differing methods of preparation such as abrasive blasting prior to coating. Depending on the material you are sand blasting, light sanding with a fine grade sand paper will smooth out the "peaks and valleys" created by the blasting process. Most powder coating companies will offer blasting at an additional cost. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing. The most important properties to consider for any item being blasted with an abrasive are chemical composition and density; particle shape and size; and impact resistance.

Silicon carbide grit blast media is brittle, sharp, and suitable for grinding metals and low-tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface finishing. Sand blast media uses high-purity crystals that have low-metal content. Glass bead blast media contains glass beads of various sizes.

Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.

Clean the base metal thoroughly. Using bead or abrasive blasting on hard metal, such as cast iron or steel, will remove mill and rust scale, dirt and foreign materials. Chemical solvent cleaning will remove any grease, oil, or paint, and light sanding can be done to finish preparing the surface. Aluminum, magnesium, and other soft alloy metals can be solvent cleaned and wire brushed, or sanded if needed.

Authored by:

Rob Chalmers