Q. How do I get rid of orange peel on epoxy and high-solids paint? I am using a Mach 1 HVLP gun with a 1.8-mm air nozzle at 35 psi. It also uses a paint pot system. —C.L.
A. Orange peel effect is a very common coating application issue. To better understand how to remedy this problem, you need to understand what causes it.
There are several things that contribute to orange peel, ranging from improper coating viscosity to chemistry. Paint is paint; it doesn’t matter if it’s CARC, automotive or furniture coating. When a coating material is too high in viscosity in relation to the tip size and pot pressure, the material will not break up correctly, resulting in a rough, non-leveling coating.
Here is a process I use every day that will help to eliminate this issue for you. First, check with your coating supplier to find out what the specific spray viscosity should be. If you have not done so, you will need to invest in a stopwatch, viscosity cup and an infrared (IR) gun. There are several different types of cups, but the most common are Zahn or Ford cups. In the lab as well as fieldwork, I typically use a #4 Ford cup and sometimes a #2 or #3 Zahn cup to check the coating viscosity. But the choice of cup is only part of the viscosity evaluation process.
Before testing the viscosity, check the temperature of the material to make sure it falls within the coating manufacturer’s temperature specifications. Think about this—the oil in a car engine on a cold winter day is very thick, but once the car runs for a while, the oil becomes much thinner. It’s the same with paint, if it is cold and out of specification, it will not flow correctly when it is sprayed, no matter what tip size you are using. It is very important that you do not add reducers or thinners to the material just to achieve the correct viscosity in spite of material temperature. This will just cause problems as it grows warmer during the process (not to mention the chemistry changes caused by such an addition).
The best rule of thumb here is to make sure your material is between 70° and 77°F (or the manufacturer’s recommendation) when checking viscosity. If the material temperature is correct, but it is still too viscous, then follow your coating manufacturer’s recommendations as to the proper solvent ratio reductions.
Once the material is specified correctly for spraying, you will next need to turn your attention to the proper equipment and application process. You mentioned that you are using a pressure pot system with a Mach 1 HVLP spray gun with a 1.8-mm tip. You’ll need to make sure the tip size is correct in relation to the coating viscosity. You can have a perfect manufacturer-recommended viscosity only to find that the material still does not spray correctly. If the tip size is too small, the material typically will dry spray; if it is too large, you’ll flood the surface. As a general rule, the more viscous the material, the bigger the tip size needs to be. The 1.8-mm tip at 35 psi is typically a good combination for medium-viscosity materials. If your material is much thicker, even with the proper viscosity you may want to go to a 2.0- to 2.4-mm tip-and-needle combination.
Pay close attention to your air and fluid pressure in relation to tip size as well. When spraying any material through a pressure pot, even with the proper viscosity material, tip size and air cap, you can still get the dreaded orange peel effect. If you drive your fluid pressure too low, the tip is essentially starved from getting enough material to properly break up at the air cap. On the other hand, if you drive the fluid pressure too high, the material will come out too heavily, resulting in over-application of the coating.
If you still experience orange peel after following the processes mentioned, you’ll need to contact your coating manufacturer for additional recommendations for possible leveling agents that can be added to the coating to help promote better leveling. They are often referred to as retarders and include N-Butyl Acetate, MAK and others.
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