Fluid retarder (also called "extender") is often mixed with a flow improver to provide the dual advantages of both improved flow control and increased drying time. We also sell one of the best flow improvers: Winsor & Newton Acrylic Flow Improver for this purpose. For your convenience and economy, we also provide a premixed "flow improver and fluid retarder" in a conveniently sized dropper bottle, which we have named Lasting-Flow™.
Lasting-Flow™ is a precise mixture of 25% flow improver, 25% fluid retarder, and 50% distilled water. This ratio is recommended by leading artists in the miniature painting field and personally tested by Miniature-GIANT staff and found to be an excellent proportion -- providing the choice combination of excellent flow control and extended drying time. For more information on paint additives, their use and advantages, see the excellent article "Let It Flow" by Darin Hlavaz.