This Graco 2-quart pressure pot system I currently use is an older 2-stage model, but gives good results with proper thinning.
This Graco 2-quart pressure pot system I currently use is an older 2-stage model, but gives good results with proper thinning.
I was having a conversation with one of my customers about spraying a conversion varnish (Krystal, from M.L. Campbell) and the problems he was having with getting it to lay down nicely after it was sprayed.
He said that what he applied is wet enough to blend together and not be rough, but that he had a lot of orange peel in the finish. After discussing the possible causes of the orange peel it became obvious that he needed to add lacquer thinner to the mix, which he did not do.
This customer is new to spraying conversion varnish, which is a two-part mix that sets up and hardens chemically like epoxy, forming a super durable finish. The information on the can talked about the 10:1 ratio of finish to catalyst, but apparently didn’t mention a thing about thinning with lacquer thinner, so he used none.
Even if it was mentioned, I assume that he was worried enough about getting the ratio correct (click here to learn how to easily get the proper mixing ratios) and not messing up the mix that he never imagined he could, or even that he should add lacquer thinner.
In this case, my customer was getting orange peel because the finish was too thick for his two-stage turbine. The kids at the finish distributor led him to believe that he shouldn’t need to add thinner, but they did not ask about the power of his spray equipment, assuming that he probably had a turbine strong enough to finely atomize the finish without thinning.
I continued to discuss the need to add thinner with my customer, and pointed out that a non-thinned finish requires more turbine power than he currently has. If he owned a 4-stage or 5-stage turbine, he could probably use the finish without thinner, but not with just a 2-stage. I speak from experience on this one, because my everyday gun is an older 2-stage model, and it requires at least a bit of thinning on almost everything I spray. I am okay with this apparent shortcoming because I am a proponent of applying multiple thin coats, as compared to fewer thick coats, which I believe are just inviting trouble.
As our conversation continued, he asked the million dollar question, “How much lacquer thinner do you add?” For me, the simple answer is, “Until it sprays good,” which is very ambiguous I know, but true. 
I have an advantage because I have sprayed more than him and I have an idea where I am headed, but I don’t truly know until I shoot a sample board with it and see how things are flowing (which I do every time before I spray the real thing). I spray a sample piece of wood standing up vertically to make sure that I can get a fully wet and flat surface with no runs or sags and to get a feel for how fast I need to move the gun to make all of that happen.
If the sample surface looks good, I move on and spray the real thing. If I have issues, it is usually because the finish is a bit thick, so I add lacquer thinner until the finish sprays smoothly without orange peel and without runs.
A viscosity cup like this Ford 4 style, available from Highland Woodworking is a good starting point.
Another, more technical way to determine the correct amount of thinner is to use a viscosity cup. A viscosity cup is shaped like a funnel and determines how thick a fluid is by the time it takes to empty the cup. A thin fluid will empty in just a couple of seconds, while a thick fluid might take 30 seconds or more. When I started spraying and used a viscosity cup, about 15 seconds was the right amount for my gun, but it will vary from gun to gun.
When learning to spray, I recommend using a viscosity cup and to follow the manufacturers recommendations. If nothing else, this will give you a good starting point from which you can make later changes and have a way to achieve consistent results. After you spray for a while, there will be less mystery, and you will know from one test shot what needs to be adjusted, even without the viscosity cup.
When my customer asked about adding lacquer thinner, I know he was worried about possibly adding too much, and after thinking about it, I don’t know that you can add too much. I can follow the logic that adding too much thinner may change the chemistry, but I mix the 10:1 ratio of conversion varnish to catalyst first and then add the thinner.
So there should still be the same amount of resin and catalyst, just with more space between them, in the form of lacquer thinner which will quickly evaporate and let the two parts do their thing. Even with other lacquer products, which includes sealers, nitrocellulose lacquers and modified lacquers, I can’t think of any time that I have ever had a problem because I added too much thinner.
I’m sure finish manufacturers would disagree and warn you to not be so cavalier about it, but I sure wouldn’t worry about adding too much thinner. Simply add enough thinner until your spray gun is able to apply a nice, even and wet film that flows out flat and dries without sagging. Even if you do mix it a bit thin, feel confident knowing that you can always compensate by moving more quickly or reducing the amount of fluid coming out of the tip of the gun.