Slow-Dri Fluid Retarder — Use this additive with Liquitex Acrylics to extend drying time. It increases transparency when floating, shading, blending, and washing colors. Use Slow-Dri Fluid Retarder in place of water before loading the brush with paint to eliminate drag and improve flow. It helps to reduce paint skinning on the palette and is excellent for liner work, transparent washes, and wet-on-wet techniques. Cleans up with soap and water.
I have used this product a fair bit. It does slow the drying time. I use mostly Vallejo Model Paints and some craft paints with it. I typically use it for blending, as that is why I want to slow drying time.
I also Use Future Floor Wax, to get a harder paint and a more flowing paint. It just depends on what I am doing. I don't think this is on the market anymore. I probably bought two 640z containers 20 years ago. I beat use 4 Oz a year...
I am not sure what your are trying to do. I am not sure about a blending medium to slo, when spraying. Can I ask why you would slow the drying time of Spraying? And does spraying mean airbrush?