From the standpoint of a professional finisher, there is a reason why finishes are usually supplied toward the higher end of 'solids content'. Every gun has a slightly different feel, behaviour, What if you were spraying with airless? You can spray some pretty thick material with that, and still get good atomization, good flow, in a production environment, airless works great with lacquer, especially with a dual orifice tip. Beyond fine tuning the viscosity to the gear, there is also intention to consider, sometimes it is desireable to have a thinner, or thicker material, sometimes for reasons that you decide. Temperature? Humidity? Toning? Retarders and reducers help fine tune for differences. Fluid tip size? Single orifice? Dual orifice? To a professional finisher, thinning is not an 'option' it is just how the material is fine tuned. Most every finish gets thinned. More thinner coats is almost always better. C
Watco and Deft are DIY products, not professional, they are made to be friendly with non professional finishers. Cardinal is not the easiest lacquer I have ever sprayed, it requires the gun to be set up well, and good technique. If everything is ideal, and you have an efficient sequence to spray an instrument, the results of each coat can be very glassy. If things are less than ideal, including, possibly, your technique, more texture in the finish will likely result. Thinning can adjust the flow, leave a glassier result. Some will say that the application does not matter, that at the end of the day you can sand anything 'flat' and polish it, which is true to a point, but, getting it glassy from the git go saves so much time. Different guns will likely require different thinning ratios. Some guns will require none. Different temperatures and humidity will each affect how the viscosity of the lacquer plays in to how it 'lays out'.
With your experience with Deft, when you start using Cardinal, or other similar lacquer, a real lacquer, you will begin to understand the difference. And, you will never look back.