To repair a crack at your exterior stucco, the first thing you need to do is to remove the stucco around the crack, to make an open area slightly larger than the size of the crack. You can do this by using a chisel and a hammer. After making sure your opening is the sufficient size, you must clean the area to be repaired first. You can do this with a brush. After that, for narrow cracks, you can use a stucco repair caulk. Just fill the crack with caulk and smoothen it as you fill the gap. For larger cracks, you need to use a stucco patching compound.