Which of the following is NOT a basic inspection principle for penetrant methods?

The option that is identified as not being a basic inspection principle for penetrant methods highlights the fundamental limitations of penetrant testing. The primary function of penetrant testing is to identify surface-breaking defects. While it can effectively reveal cracks or other discontinuities that are open at the surface, it cannot detect subsurface defects. This makes option C particularly relevant since subsurface discontinuities involve flaws that do not reach the surface, and penetrant techniques are inherently unable to reveal such conditions.

In contrast, the first three options represent accepted practices and principles associated with penetrant testing. Indications that glow under black light (first option) refer to the fluorescence of certain penetrants, which assists in visualizing defects. The second option discusses the various methods of applying visible penetrant, demonstrating the versatility of this inspection technique. The fourth option touches on the prerequisite of preparing the test surface by removing paints, which is essential for an unobstructed application of the penetrant and accurate results. Thus, this distinction in capabilities underscores why option C does not align with the basic principles of penetrant methods.