Thenrie,
Applying paint is one of those subjects that can turn into very long draw out debate. There are many opinions as to what is right and wrong. This posting is probably going to stir up a HUGE can of worms, but you are requesting advice, so I will give it.
First, if you are spending the money for a better than spray can finish, you should only buy paint from a paint supplier that mixes and tints to your request. They can match any color out there. When you purchase paint from other places, (even IH & CC dealers) you do not know exactly what you may get until you open the can. Or, how long it has been on the shelf.
The average shelf life on many paint materials is 6 months. Not only will a paint supplier freshly mix the paint exactly like you ask, they will also be able to answer any questions you may have about application. If you find they don't want to answer your questions, go to a supplier who will. The extra cost paid for the material at a paint specific store is easily made up by the knowledge the supplier has about their product.
That being said, you have already purchased some paint so let's start there. There is no formula for the amount of thinner to add to paint. It is all based on it's viscosity (thickness) and the type of application you plan to use. In your case, HVLP. Viscosity is measured with a viscosity cup.
The reason for this is because so many things affect the viscosity of the material. The air temperature, humidity, pigment content & solvent content are just a few of the factors that affect a paints viscosity. In an industrial application setting, viscosity readings will be taken many times a day. Especially when opening new containers of paint. If you follow a formula of "x amount of solvent to x amount of paint" you will never end up with the same thing twice.
A viscosity cup is a cup (imagine that!) with a precise sized hole in the bottom. After thoroughly mixing the paint (before adding any solvent), you dip the cup in the paint then hold the cup steady so the level of the paint is the same height as the lip (top) of the cup. With a stopwatch in hand, you quickly, but steadily raise the entire cup out of the paint. At this instant, start the stopwatch. A steady stream of paint will flow from the hole in the bottom of the cup and back into the container. As soon as that stream breaks and becomes a drip, stop the stop watch and note how many seconds it took for this to happen. Repeat at least twice to verify similar readings.
Thinner paints take less time to empty from the cup. Thicker paints take longer. Record how many seconds it took for the cup to empty, and what the air temperature and humidity level are. You now have a reference to go back to for applications at later dates.
There are many types of viscosity cups out there, but the most popular is what we refer to as a #2 Zahn. Even when you start referring to a specific size cup, there are dozens of choices as to which #2 Zahn you use. Since they are a precision measuring device, prices and quality can vary. Shop around.
Your paint supplier should be able to tell you how thick the paint should be for proper application. Again, since you already have paint, you can go with a "rule of thumb" that HVLP guns spray best with a viscosity of 14 seconds or less, normally not below 10 seconds, in a #2 Zahn cup. Add small amounts of solvent (thinner) as needed. If your application time will take longer than a hour, (lots of parts) periodically check viscosity again.
Without knowing what type of paint you currently have, I can not recommend which thinner to use.
Sorry for this overly detailed post, I hope I didn't put you to sleep!
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