Nondestructive surface crack detection is an important issue in many industrial, manufacturing and structural environments. Although there exist several standard nondestructive techniques, each possesses their respective advantages and disadvantages. Recent research activities in using open-ended microwave sensors has proved to be a viable practical approach. Microwave techniques offer many advantages including the fact that they may be used in a noncontact fashion while detecting exposed, filled and covered surface cracks. This paper explores the noncontact characteristics of employing an open-ended rectangular waveguide sensor at K-band. The results show the possibility of detecting thin and long machined slots/ cracks on aluminum plates at standoff distances of several millimeters. In addition, the results show that these cracks/ slots are readily detected when exposed, filled with rust and covered with several layers of common paint. A discussion of the findings is also presented.