Depending on the length and straightness of the crack one approach would be to use a dremmel and actually open up the crack a little more, drilling a small hole at each end is also helpful. So now you have a small hole at each end and a wider opening in the crack. Run Blue painters tape along each side of the crack, fill with a paintable Caulk, flatten out running a small putty knife over the tape flattening out the caulk, remove tape. After everything is dry tape off each side of the filled crack, go a little wider than the actuall crack, I am talking about 1/16". Spray with best matching paint, remove tape.....