Dear Joe,

We are starting to manufacture and sell our patent pending, ergonomically adjustable computer workstations. We are interested in the powder coating process for wood substrates such as high-density fiberboard (HDF) that have an even core and face. Do powders for wood substrates still need refrigeration? We would also like to find a powder coater of wood substrates in Northern California or near Reno, NV, that does piece work.
Norman Allen, Infinity Station

Dear Norman,
Powder coating processes for wood substrates are gradually popping up all across the landscape. Some are big OEMs, while others are small to medium jobcoaters. Some have ultraviolet (UV) cure capabilities, whereas others rely solely on thermal cure powder technology. In your neck of the woods, there is a coater called Do-Able Products in Chino, CA, that can powder coat wood.

It sounds like you are also interested in the possibility of eventually installing your own line. You will want to contact a few key powder suppliers and equipment manufacturers. A good starting point would be Finishing Today's Buyers' Guide, which is available online at www.finishingtodaymag.com. The 2006 edition will be published in November.

As for the handling of powders designed for application to wood, refrigeration is not normally required. However, careful handling and transport is strongly recommended, since powders for wood usually have lower melt points than conventional powder coatings. For instance, instead of having your vendor ship this type of powder to you on a Friday during the middle of a scorching summer, have it overnighted to you earlier in the week to minimize the possibility of having the product sit at a shipping depot in a dark truck over the weekend. Similarly, don't store a wood powder next to the cure oven in your plant. I recommend air conditioned storage as opposed to refrigerated handling.

Good luck with your endeavors. It's always refreshing to hear of someone pushing back the frontiers of coating technology.

Dear Joe,

We are considering changing to powder coat finishing from two-part polyurethane for our aluminum shutters. We currently paint over existing factory finished polyester coatings (.007 in. thick) that have been baked on at the factory. I seem to be getting conflicting answers as to whether this will work due to the possible insulating factor of the factory paint. One person says that the factory finish must be conductive, and another says that you just need a good ground. What is your opinion?
Steve Sutton, Willard Shutter Co.

Hello Steve,
Powder can be applied over existing finishes. A good example is the clear topcoat that BMW applies to its 5 and 7 series automobiles in Germany. Those cars already have an e-coat, primer surfacer and color basecoat before seeing the final coat of powder. That being said, you still might have trouble powder coating over .007 in. of polyester. Even though the polyester has been baked in the factory, can it survive another possibly higher temperature bake for the powder? And will the powder apply evenly on the somewhat insulated polyester surface? Another issue is whether the powder will achieve acceptable intercoat adhesion to the polyester.

Ideally, you should perform a test by applying the powder of choice to a non-critical scrap part. Make sure the part is adequately grounded to achieve the best electrostatic attraction of the powder, and reduce the current on the gun. (If the gun doesn't allow for current reduction, just reduce the charge to about 50 kVs.) The polyester surface should be clean and free from oils and contamination. I recommend that you wipe the surface with a weak solvent such as VM&P naptha prior to powder application. If you can use a scrap part, test the powder coat adhesion after the part cools by making a cross cut with a razor blade. Apply packing tape to the cut, and pull. The powder coating should adhere 100%.

I hope that this information helps. Let me know how things work out for you.

Dear Joe,

We've been having a nightmare with craters in our powder shop over the last few months. We've checked all of our equipment, including the air compressor, filters and air dryer. We only spray polyesters in our shop. However, one of our neighbors in our industrial park applies a silicone-based military coating. I'm convinced the overspray mist from our neighbor is causing our trouble. Am I correct in my thinking, and what can I do to remedy this situation?
Todd

Dear Todd,
Serious contamination of powder coatings can occur from contact with silicone-based paints. The defects can take the form of craters or "fish eyes" in the finished coating film. The contact of the silicone with the powder can occur through a number of mechanisms. Mist from a finishing operation involving silicone-based paints can carry long distances and become entrained in the ambient air of a powder coating operation. This mist constitutes airborne contamination. The mist of silicone based paint can also contaminate a powder coating finish by depositing itself on the surface of parts before or during the powder coating process. This problem is referred to as surface contamination. Furthermore, the mist from a silicone base paint can be deposited on air handling equipment surfaces such as filters and thereby be distributed to the ambient air, compressed air and/or finishing surfaces.

Silicone-based products, including silicone-based paints, cause craters in powder coatings due to the inherently large difference in surface tension between the silicone-based paint and the powder coating. Craters form when a region of very low surface tension resides within the melted film of a powder coating. The surface tension of silicone-based paints is typically measured at 15 to 20 dynes per centimeter. The surface tension of powder coatings is typically 40 to 50 dynes per centimeter. This is a large enough discrepancy to cause fish eyes.

It only takes a very little concentration of a silicone-based contaminant to cause cratering in a powder coating. Laboratory studies have shown that concentrations of silicone-based products as low as 0.001% can cause widespread cratering.

Remediation of silicone contamination of a powder coating system requires exhaustive measures. If the source of the contaminant can be identified, the first action is to isolate the cause. The next steps involve the painstaking procedure of cleaning the entire finishing shop. You should thoroughly clean all air handling equipment, including compressed air systems, booth exhaust and oven air handling systems. You should also thoroughly clean all powder handling and application equipment, including guns, hoppers, hoses (these should be discarded if possible) and booth surfaces. The use of denatured alcohol is recommended to clean plastics parts. It can dissolve the silicone residue while leaving the plastic intact. A test application can be conducted at this point. If contamination is not eliminated, then all surfaces in the shop should be thoroughly washed with an industrial cleaner.

Contamination of a powder application system is a very serious problem due to the minute concentrations of silicone that can wreak havoc on the powder finish. Diligent root cause analysis is vital, as is thorough cleaning of the entire powder application system.

Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to askjoepowder@yahoo.com.