Painting over partially dried acrylics is a tricky business and can create some mixed results. Applying a wash over semi-dried paint can lift patches of colour, and pull out the remaining solvents prematurely causing the surface of the paint to form craters, lose clarity or dry to a more matte finish.
When a thin acrylic layer is dry to the touch, more layers can be applied with no adverse effect. The new layers will not reactivate the underlying paint, and luminous glazes can be accomplished using this method. When layering in thicker quantities, it is essential that the paint be cured before applying more layers. If it is not, water and other solvents can become trapped in the partially dried acrylic paint, clouding the acrylic film.
The most prudent approach to layering and over-painting is to give the initial coat ample time to dry before applying more paint.