If you wish to slow the drying of acrylic on a painting so that you have more time to blend, you can mix in a very small amount of retarder, an additive designed to retard the drying process. Or you can use one of the slow-drying acrylics like Golden Open or Atelier Interactive. This will also keep the paints moist for longer on your palette.
Ways to slow the drying time of acrylic paint.
Updated over a week ago