How can I smooth and repair cracks in a plaster wall?
This is the step by step process I use.


Dig out all the loose material from each crack. Blow out debris with compressed air. 
Brush a bonding agent into the cracks and allow it to dry. 
Force setting type joint compound into cracks and while still wet over lay the cracks with fiberglass mesh tape and press it flat into mud against surface. 
Let it dry, then knock off any lumps or bumps and apply a second coat, feathering all edges. 
Let dry and sand area to smooth it out. 
Apply a coat of topping mud to perfect your patched area making it as smooth and trowel mark free as possible. 
Let dry then lightly sand remove dust and prime patched area. 
After it dries repaint the walls and ceilings with a quality acrylic paint.