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9 Janvier 2021
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When the finish is the right thickness, it will spray well.
With one simple tool you can easily measure and adjust the thickness of your finish to make sure it will spray well. Think of thickness in terms of being thin and runny like water or thick and syrupy like honey. This flow property of a fluid is called viscosity – it's considered low when the fluid is thin, and high when it's thick.
When the finish is too thick for the spray gun you're using, it will come out slowly (if at all) and will spatter – creating little visible droplets that produce a bumpy finish. But when it's the right viscosity, the spray pattern will look like a fine mist and the resulting finish will be smooth. Although some spray guns can spray thick coatings (e.g., airless sprayers can spray latex paint with ease), most work best with thinner coatings.
Making sure the viscosity is right is the first step of spray finishing. You'll need a viscosity cup, a stop watch, some finish, and some thinner.
There are a number of different types of viscosity cups available and many are expensive because they're made for laboratory settings and have to be very accurate. Two types, Ford and Zahn, are most common for measuring coatings. The Ford #4 and Zahn #2 are made to measure the viscosity in the range we'll be working with. For spray finishing we don't have to have a laboratory grade cup – we just need one that does a good job. The white plastic cup pictured on the right is a good example – it's a Ford #4.
Watch the stream of fluid closely for the first break.
Dip the viscosity cup below the surface and lift straight up. Seasons 1 2.
Measuring viscosity is easy. To get started, stir the finish to make sure it's mixed well but don't create bubbles. If you do, wait until the bubbles dissipate before you measure. Once mixed well, dip the cup into the coating just below the surface to fill it (picture on left) and keep it level. Start the timer or stopwatch at the exact time you lift the cup straight up. Let the fluid flow out the hole in the bottom (picture on right) and watch the stream closely until you see the first break then stop the timer. The number of seconds it takes for the fluid to run out of the cup is the viscosity measurement.
The temperature of the finish will affect the reading. As the temperature goes down, coatings get thicker and vice versa. In some industrial systems, the coatings are warmed before spraying which lowers the viscosity. The temperature has to be controlled precisely as excessive heat can harm the properties of the finish. Always store and use finishes in accordance with the manufacturer's requirements – that information should be on the can and/or the Product Data Sheet (PDS).
Here's an excerpt from the Product Data Sheet (PDS) for a white conversion varnish called AcromaPro (formerly Becker Acroma) Matador. According to the data sheet, the viscosity of this this paint is between 155-185 seconds in a Ford #4 viscosity cup at 73°F. If the temperature is lower, the viscosity will be higher.
Further down in the same data sheet, it tells us the recommended viscosity to spray the paint using various types of spray guns. It gives us a range of 17-27 seconds to work with. Straight out of the can the paint is much thicker than this (155-185 sec.) – we will have to add thinner until the viscosity is low enough to spray well. The goal is to use the least amount of thinner needed to get a well atomized spray pattern. You'll need to experiment a bit to see what works best with your spray gun.
To get started, measure out a small amount (e.g., pint) of the finish you'll be using and add the appropriate thinner in 5% increments until the viscosity is in the 17-27 seconds range. Keep a record of the percentage of thinner needed so you'll have it for the next time you need to mix some of that finish. Once you're in range, load the spray gun for some test spraying. If the finish isn't atomizing as well as you'd like, measure out more thinner, mix it in, and test spray again. When it's working well, measure the viscosity to see what works best with your spray gun and keep it for your records. In the future you'll be able to thin finishes to that value and get consistent results.
You may run into a case where the Product Data Sheet (PDS) gives you viscosity data for a different type of viscosity cup. For example, the sheet may list the recommended viscosity as '20-22 seconds, Zahn #2' or something similar. Zahn is a type of viscosity cup, and #2 is one of the cups they make with a specific size fluid opening in the bottom of the cup.
If you have a Ford #4 viscosity cup, you need to convert the time range from the Zahn #2. To do this, just look at the viscosity conversion chart below and look up 20 seconds in the row by Zahn #2. Then go up a couple lines to the Ford #4 entry 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.
Using your Ford #4 cup, the range you want is 14-18 seconds which is the same as 20-22 on the Zahn #2 recommended on the data sheet in this example.
Here's a quick demo showing the process of measuring the viscosity using some green spray paint. The operator starts the timer as he lifts the cup and stops it when the stream of fluid coming out the bottom first breaks.
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