Which of the following is NOT involved in the visible penetrant application process?

The visible penetrant application process is a method used primarily in non-destructive testing to detect surface-breaking defects in materials. This process typically involves several steps where a liquid penetrant is applied to the surface of a part, allowed to penetrate any defects, and then excess penetrant is removed to prepare for inspection.

In this context, brushing, spraying, and dipping are all valid methods of applying the penetrant. Brushing allows for thorough application on uneven surfaces, spraying offers a fast and even coverage, and dipping provides a complete submersion that can ensure full penetration into any cracks or surface defects.

Heating, however, is not part of the typical visible penetrant application process. While heat may be used to aid in the drying phase or to enhance the penetration characteristics in some materials, it does not serve as a method for applying the penetrant itself. Instead, the focus is on the application methods that allow for the penetrant to enter surface flaws. Consequently, identifying heating as not being involved in the application process is accurate and aligns with standard practices in non-destructive testing.