[en] The advantage of applying liquid penetrant inspection (LPI) on almost all materials is well known. It is relatively non-costly method for detection and interrogation of open-to-surface discontinuities on non-porous surface. The most critical part in the inspection is the surface preparation. The surface shall be free of oil, grease, water, or other contaminants that may prevent penetrant from entering discontinuities. After years of applying penetrant on the surface of samples used for training purposes and post cleaning was not done properly, most of the discontinuities that were initially found in the samples were no more detected by LPI. These discontinuities were fully covered with residual of penetrant that failed to be eliminated by appropriate method of post cleaning. The paper describes ways to reveal the initial discontinuities in the sample by methods of neutron radiography and conventional radiography. (author)