Mud cracks appear when you apply a thick coat of paint to hide an improperly prepared porous surface. Some poor quality paints do not hide the underlying stains, and therefore there is a tendency to apply a thicker coat, resulting in mud cracks. At times thick coats are left at the borders of the painted area. On drying, they may develop cracks that spread to other areas.
Do not repaint over the cracked areas as you will see worse results on drying. Also do not use the same type or quality of paint that you used earlier. You will have to remove the entire paint film. Scrape and sand the area thoroughly to remove the coat of paint with mud cracks. Coat with a good primer and allow it to dry. Use a good quality latex-based paint with good hiding capabilities. Follow the manufacturer’s instructions regarding the spread rate.
A good quality latex paint does not break on drying as it has the right solid content and the right viscosity. It is usually more flexible than oil-based or alkyd paints and prevents mud cracking.