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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.

Viscosity Cup

A viscosity cup (shown on left) is used to measure the thickness of the coating (finish, paint, etc.) being sprayed. The first step when getting ready to spray is to check the viscosity of the material and thin it if needed (check with the coating manufacturer for thinning limits). Spray gun manufacturers have a recommended viscosity for each of the spray guns they make, and your spray gun will work best if the viscosity is right. If you always spray the material at the same viscosity, you will always get the same atomization and consistent results. You can thin just the quantity you plan to spray today, or you can thin the whole container. But the temperature of the material affects the viscosity, so keep it at the same temperature all the time if possible. If the finish is 65 degress when you thin it and 85 degrees when you spray it, it will be thinner when it's warmer and will get runs and sags if you don't spray lighter coats. Finishes and paints are thicker when cool, and thinner when warm.

Filling the Cup

Using the viscosity cup is easy. Dip the cup into the coating just below the surface to fill it to the rim (picture on left). Using the second hand on a clock or watch, mark the exact time and lift the cup out and let the material flow out the hole in the bottom (picture on right). Watch the fluid stream closely and mark the exact time it stops. The number of seconds it takes for the fluid to run out of the cup is how the viscosity is measured.

Fluid Pouring Out

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.

POISE	0.10	0.15	0.20	0.25	0.30	0.40	0.50	0.60	0.70	0.80	0.90	1.00	1.20	1.40	1.60	1.80	2.00
CENTIPOISE	10	15	20	25	30	40	50	60	70	80	90	100	120	140	160	180	200
Ford #3			12	15	19	25	29	33	36	41	45	50	58	66
Ford #4	5	8	10	12	14	18	22	25	28	31	32	34	41	45	50	54	58
Zahn #1	30	34	37	41	44	52	60	68
Zahn #2	16	17	18	19	20	22	24	27	30	34	37	41	49	58	66	74	82
Zahn #3											10	12	14	16	18	20	23
Zahn #4												10	11	13	14	16	17
DIN #4	11	12	14	16	20	23	25	26	28	30	34	38	42	45	49	52
ISO #4		17	23	34	51	60	68	74	82	93

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.

Wet Mil Gauge

Using the Gauge. When you spray finishes, the wet mil gauge helps you get consistently good results. It's about the size and shape of a credit card and has a number of cut-outs on each side that are progressively shorter by 1 mil in length (about the thickness of a sheet of paper). In the picture, you can see the numbers 1-6 at the bottom, 7-12 at the top, and 14-80 on the sides. Each of these tabs that are cut into the gauge are labeled to indicate how far they are from the surface you set the gauge on. To use the gauge, you first spray a flat surface and immediately set the gauge on the wet finish. It's easier to do this on horizontal surfaces. Then you lift the gauge and count the impressions it left in the finish.

Impressions From Gauge

Reading the gauge. When you lift the gauge, you'll see a number of impressions in the wet finish (they disappear). Count the number of marks, subtracting the two on the ends where the gauge feet contact the surface. The number of impressions will tell you how many of the mil tabs touched the finish and that tells you how thick you sprayed the finish. Most finishes we use on furniture are sprayed in the 1-5 mil range, though some go on thicker. In the photo, ignore the two impressions on the ends and count the remaining marks. This finish was applied at 5 mils (ignore the marks on each end). After counting the marks, use a clean cloth to wipe the gauge clean. If you allow finish to build up on the measuring tabs, you'll get incorrect readings from the 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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