What should be done if a liquid penetrant test is performed on a coated surface?

When performing a liquid penetrant test on a coated surface, it’s crucial that the surface be prepared correctly to ensure accurate results. The correct approach is to completely remove the paint or coating because any layer of paint can interfere with the penetrant's ability to seep into defects, such as cracks or voids, within the underlying material.

Liquid penetrant testing relies on the penetrant materials being able to enter flaws in the substrate, and coatings can create barriers that hide these imperfections. If the coating is left intact, it can trap the penetrant, leading to either false indications or missed defects entirely. Only by thoroughly cleaning the area and removing any coating can the test be carried out effectively, ensuring that all surface irregularities are accessible to the penetrant.

In contrast, simply scraping off the coating may not be enough to eliminate all residues, and ignoring it entirely would likely render the test ineffective. Likewise, not conducting the test does not provide any insights into the integrity of the substrate, which is why complete removal is necessary for the integrity of the testing process.