To avoid any compatibility problems, it’s often best to employ the same brand of paint and thinners. Through trial and error, you’ll probably find a few brands of thinner that work well with other brands of paint but, when starting out, it’s best to err on the side of caution. Virtually all hobby paints require thinning before spraying. Some, such as Com-art and Vallejo Air acrylics, or Alcad2 enamels, come pre-thinned. However, once they’ve been used a few times such paints tend to thicken. I’ve found it best not to concentrate on specific ratios of paint and thinners, as performance differs wildly by brand, batch, shelf life, storage conditions and ambient temperature. It’s much better to gain an appreciation of the desired consistency to suit the airbrush; in this way, any type of paint or varnish can be thinned with a greater chance of success.
Be sure to choose the right type of thinners to match your paint, studying the information on the label and heeding any safety warnings. With solvents, always avoid contact with skin and eyes.