Which of the following is NOT a generally accepted method for applying penetrants?

Applying penetrants in non-destructive testing is primarily aimed at revealing surface defects in materials. The generally accepted methods for applying penetrants include spraying, dipping, and brushing, as each of these methods ensures an even and complete coverage of the test specimen, which is critical for accurate results.

The method of rubbing the penetrant over the surface is not generally accepted because it can lead to inconsistent application. Rubbing may not adequately saturate crevices or surface defects where penetrants are supposed to seep in for effective inspection. This inconsistency can lead to ineffective testing and the likelihood of missing critical flaws. Therefore, the other methods, such as spraying, dipping, and brushing, are preferred as they promote uniform penetration and enhance the reliability of the inspection process.