What colors and paint lines can I use with my IWATA airbrush?
In general, The Heavy Body, High Loads, Fluids and Airbrush Colors can all be used in an airbrush with the appropriate thinning/extending. Here at GOLDEN, we grind our pigments by milling, and each pigment is ground as fine as possible without destroying its structure. With that in mind, certain colors will not spray as well as others. For instance, earth color pigments are a coarse chunky particle you can not mill too tight or it will break down. Therefore, smaller airbrushes, like an Iwata HP-C will have a tougher time spraying these pigments.

That being said, you can use any of these paint lines for spraying, but the thicker ones will require more thinning, and therefore the sprayable mixture is effected by how much thinning was done. Heavy Body paints will require in the neighborhood of 7 parts Airbrush Transparent Extender to 1 part color; High Load Acrylics will be 4:1; Fluids will be 1:1 and Airbrush Colors, of course, will be ready to spray. It is not uncommon for artists to use a combination of paint lines for their work, as they are the same pigments in each line and they are all made for use in fine art applications.

What is a good mixture with GOLDEN Fluid Acrylics to do stippling? The 1 to 1 mixture does not work with the Iwata PC.
Try using the Fluids as is or slightly thinned with water. The Airbrush Medium is meant to reduce spattering, so adding it will improve atomization. Try turning your pressure down a bit. We also stipple by doing one of the following:

1. Hold the tip of a plastic knife in front of the nozzle and have the paint spray onto it first. It will collect and spatter off in a larger stipple pattern.
   
2. Use masking tape to make a long cone (1" - 1.5") on the end of the airbrush. The paint will collect on the tape and spatter as it comes out. We use this one if there is a lot of stippling to be done. It gets messy to clean up, but can be done with one hand instead of two.

We have not had much luck spraying your Heavy Body paints (along with your Airbrush Medium). We will try a 1:7 ratio mix of Heavy Body paint to Airbrush Medium as per your Web site suggestion. Do you have other words of wisdom pertaining to this subject?
It sounds like you would be better off trying to spray the Fluid Acrylics, rather than the Heavy Body. They have the same pigment and pigment load as the Heavy Body Acrylics, but are much thinner. A 1 to 1 ratio should spray well whether you use an airbrush or larger equipment. The premade, ready to spray Airbrush Colors should be fine for your use as well.