"Viscosity & Wet Mils"
When you use spray equipment, the material you are spraying has to be the right thickness for the spray gun or spray system you're using. The thickness of the coating is called viscosity. Check with your spray gun/system manual or the manufacturer to find out the proper viscosity for the spray gun you're using.
If it's too thick, the finish won't atomize well or may not even come out of the spray gun. Poorly atomized coatings (clear finishes, pigmented finishes, paint, etc.) create a rough surface and look bad; not to mention you probably have to move very slowly to get anything close to even coverage. When the material is properly atomized, it looks like a fine mist and leaves tiny droplets on the surface being sprayed. These tiny droplets flow together and make a smooth, level film.
The first section below describes how to adjust the viscosity of the coating you're spraying so it's just right for the spray equipment you're using. The second part describes how to spray the right amount of material. By using a wet mil gauge, you can measure how thick you spray the finish on the surface. If you spray too thick, you will get runs and/or sags in the finish. And if you don't spray thick enough, you will get an inconsistent finish with rough spots, dry spots, a hazy look, or a number of other problems.
At the end of the wet mil gauge discussion there is a link to an article that describes spray patterns to use in different spraying situations, including inside cabinets, very large surfaces, and irregular surfaces. Whenever you spray inside any type of cabinet, it will make your job a lot easier if you leave the back off and spray it separately.
As an example, the manufacturer of your spray gun may list the recommended viscosity as "20-22 seconds, Zahn #2" or something similar. 20-22 seconds is the time range for the cup to empty. Zahn is a brand of viscosity cup, and #2 is one of the cups they make with a specific size opening.
If you have a Ford #4 viscosity cup, you need to convert the time range for the Zahn to the
time range for the Ford. To do this, just look at the viscosity conversion chart below and look up 20 seconds under Zahn #2.
Then go over to the Ford #4 column and you'll see it's 14 seconds. The higher number,
22 seconds on Zahn #2, is the same as 18
seconds on the Ford #4.
So on your Ford #4 cup, the range you want is 14-18 seconds (same as 20-22 on the Zahn #2 recommended by the spray gun manufacturer in this example). Use the chart to convert the actual numbers the manufacturer supplies to the viscosity cup you have. Try to thin the material the same amount every time within this range for the most consistent results.
If you don't have a viscosity cup, you can get one from any finish supplier. Spraying finishes and paints that are always the same thickness will make your life so much easier you'll wonder how you ever managed without it.
"Using a Wet Mil Gauge"
The manufacturer of the finish you use most likely has a recommended mil thickness for spraying (if it's a spray finish). By using the wet mil gauge, you will get an "eye" for how the finish looks when you spray it at the recommended thickness after using the gauge for a short while. If you have thinned the finish to adjust its viscosity, you will have to spray thinner coats to avoid runs and sags. With some finishes there's also a maximum number of coats you should apply - each one at a specified thickness.
A common problem when learning to spray (and for experienced sprayers as well) is avoiding the tendancy of spraying too thick (especially on horizontal surfaces). On vertical surfaces you will get sags and runs, and on horizontal surfaces you'll get trapped air bubbles. Using the wet mil gauge to learn how the finish looks when you spray it the recommended thickness helps you to avoid these problems very quickly.
For tips on patterns to use when spraying a variety of surfaces, click on this link - Spray Patterns.
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