The paint will continue to get tougher as the moisture passes through the paint. For exterior use, a few weeks without heavy precipitation is recommended. Any finish can be used to top coat real milk paint. Oil based finishes such as polyurethane will darken the color. Test to see result.
Crackling is the process of laying a faster drying finish over a slower drying more flexible finish.
In this case we will be using hide glue. Now the process. First paint the color of Real Milk Paint you want to show through the cracks. You may want two coats of this color to be sure it is totally opaque . Brush the hide glue over the dry milk paint.