Typically, when spraying Acrylic laquer, there are different formulations of Thinner for different temperatures, and very very rarely is retarder needed. NOTE ACRYLIC laquer, not Butyrate or nitrate here, I painted lots of cars with Acrylic laquer,, in all kinds of weather and used slower reducer for hot weather. So the answer is to go to a auto paint supplier that still sells Acrylic laquer, and get slower reducer for the product you are spraying, only If that failed would I risk introducing retarder into the mix.