A READER RESPONDS:

Dear Joe,
We read your response to the question about in-line rack and hook stripping (see below) and were surprised to see only one technology mentioned or recommended — thermal burn-off. It has been our experience that any thermal stripping method will require some form of secondary cleaning operation to achieve truly clean hooks and racks. With the heat intensity required to thermally remove cured coatings in-line, there is also a tendency to vitrify or fuse the inorganic portion of the cured coating. This reaction can form an electrically insulating layer on the racks, potentially compromising good part grounding and transfer efficiency. As you recognized, the use of thermal methods is restricted to heavier racks due to the potential for thermal distortion.

A thermochemical (molten salt) stripping process provides complete removal of cured coatings (both organic and inorganic constituents) in a matter of seconds and at lower operating temperatures than thermal-only systems. Acting as both a heat source and heat sink, parts are quickly brought up to the stripping temperature, and any heat generated by the stripping process is absorbed by the chemical bath. Thermochemical stripping techniques also operate at normal line speed, allowing the stripping operation to work day in and day out, while parts are still being coated on the line. There are no inorganic residues or secondary cleaning operations required other than a simple water rinse.

--Jim Malloy, Vice-President – Technology, Kolene Corp.

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Dear Joe,
We recently installed a powder line for automotive parts. Now we would like to install an in-line stripping process for our racks. Can you help us?

--Jeremy Chung and American Showa, Inc.

Hi Jeremy,
Your best choice is a burn-off system. In this process, the coating actually ignites and burns off the rack. These systems operate in-line at temperatures ranging from 1000 to 1200ºF. The high temperature is needed to burn the accumulated coating in a relatively short period of time.

It’s important to use this process only if the racks are sturdy enough to take the high heat; light-gauge racks will invariably warp under these conditions. The other important consideration is ensuring that the coating residue is adequately removed from the racks prior to hanging new parts on the line. Otherwise you run the risk of contaminating parts with bits of ash. Residue removal is usually accomplished by pressurized water streams.

The best place to locate suppliers of this technology is to peruse Finishing Today’s Buyers Guide. You can access it on our website by either clicking on the “Buyers Guide” button in the left-hand column or performing a search using the fields located at the top of our website. There are a number of companies listed that specialize in burn-off stripping technology.

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Dear Joe,
Can powder coatings be used on wood for picket fences? I think it would be cheaper than vinyl. Let me know if this is possible. Thanks.

--Joe Schirmer

Hello Joe,
Indeed, you can powder coat wood for picket fences. The seminal issue is what type of wood should be used to build the picket fence. Natural wood types such as pine or cedar, common picket materials, have a high moisture content and are difficult to powder coat because of the temperatures required to cure the powder. Pressure treated wood, also a prime picket material, is out because of its intrinsically high volatile content, which will cause severe blistering during the powder thermal process.

Your best bet is to use a specially treated medium density fiberboard (MDF). Engineered boards such as these are normally used for indoor applications; however, some manufacturers have begun incorporating water- and mold-resistant additives to make the boards more durable.

Once you find a good source of durable MDF, you’ll need to find a supplier that specializes in powder coatings for MDF. For MDF pickets, I recommend that you stick with a thermally cured powder as opposed to an ultraviolet (UV) cure type. UV cure systems require line of sight for complete cure, and the three-dimensional aspect of a picket will make curing all surfaces difficult. Outdoor, durable, thermal-cure, polyester-based powders are available for MDF coating.

Good luck with your fence project.



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Dear Joe,
What causes powder to “spit” out of the gun? About every three months or so, one of the guns in my automatic spray booth starts spitting. It only happens with one gun at a time. Some of the parts that have spit marks cannot be salvaged and need to be scrapped. How can I prevent this problem?

--Desmond Glynn, Columbia Lighting

Hello Desmond,
Spitting occurs when a dense slug of powder exits a spray gun, usually causing an irreparable blob in the cured finish. The predominant causes include:

• A constricted feed hose
• Damp powder
• Inadequate air pressure
• Poor powder fluidization
• A clogged powder venture

In your situation, the culprit probably isn’t damp powder or poor fluidization, as this would cause a spitting with more than one gun. Chances are you have a problem with your feed hose or venture, or inadequate air pressure to the gun. Check your feed hose for kinks or collapsed areas, and replace it with a new hose if necessary. Pull your venturis, inspect them and clear them with compressed air. Replace them if they are excessively worn. Check the compressed air that is flowing to your gun by detaching the hose at the gun and observing the air pressure while triggering that gun. If the air pressure is too low, adjust it or investigate the cause.

The best way to avoid this intermittent problem is to regularly inspect and maintain your powder application equipment and system. Depending on the volume of powder sprayed, you should perform maintenance weekly, daily or every shift. Maintenance should include the following steps:

• Disassemble, clean and inspect every spray gun
• Check each gun for voltage
• Detach and clean every hose
• Disassemble, clean and inspect venturis (powder pumps) and pick-up tubes
• Empty, clean and inspect hoppers
• Inspect and measure the travel of the gun movers, if applicable
• Lubricate the moving parts of the gun movers with a non-silicone lubricant
• Check the hangers for acceptable ground
• Inspect the air supply for cleanliness
• Inspect and maintain the air filters and air cleaner (desiccant or dryer)
• Measure the booth face velocity
• Inspect and clean all booth surfaces and the application area
• Inspect and test all safety devices (arc detectors, etc.)
• Inspect and test all booth filters, measuring the pressure drop across both the clean and dirty sides
• Record and maintain a log of all gun settings, environmental conditions (temperature and relative humidity), preventative maintenance and repair, equipment problems, and resolutions

Good luck rectifying this problem. Let me know if you find a non-obvious cause.



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