Which of the following is a recommended practice for preparing the surface of a part before penetration testing?

Preparing the surface of a part before conducting penetration testing is crucial for obtaining accurate and reliable results. The recommended practice of cleaning the surface to remove dirt, grease, and other contaminants ensures that the liquid penetrant can effectively enter any surface cracks or defects present. Contaminants on the surface can inhibit the penetrant's ability to fill these voids, potentially leading to false readings or undetected flaws.

This surface preparation step is vital for ensuring that the testing process is effective. If the surface is not clean, defects may remain undetected, compromising the integrity of the part being tested. Proper cleaning typically involves using solvents or cleaners designed specifically for this purpose.

The other options, while they may serve specific purposes in different contexts or applications, do not align with standard practices for surface preparation in liquid penetrant testing. Coating with wax or applying a sealant would obstruct the penetrant from accessing potential defects, while roughening the surface could introduce more contaminants and alter the characteristics of the surface in a way that is not conducive to effective testing.