In addition to reducers, other chemicals you may encounter are retarders and accelerators. A paint retarder, very simply, is a solvent that slows the drying time of the paint. Retarders are added in proportion to the directions on the paint can. Retarders are generally used in high temperature or high humidity conditions. Accelerators have the opposite affect.
Accelerators speed up the drying time.
Accelerators may be required to help the drying process in cool temperatures. Proper lighting is absolutely essential when painting. For the best results, try to position your lights 45 degrees to the surface being painted. By looking into the glare of a light you will detect heavy and light areas of material. Heavy areas will often result in sags or runs.