Why is liquid penetrant testing considered less reliable following paint removal from welds?

Liquid penetrant testing is considered less reliable following paint removal from welds primarily due to the potential for partial smearing of the weld surface metal. When paint is removed, especially through mechanical means, there is a risk of smearing the surface of the weld, which can close off any surface-breaking defects. This smearing can create a barrier that prevents the penetrant from properly entering and being drawn into cracks or other discontinuities in the weld. As a result, the effectiveness of the testing is compromised because the penetrant cannot accurately reveal flaws that may be present.

While other factors, such as paint residues and additional cleaning steps, may also impact testing reliability, they do not address the critical issue of how surface preparation can alter the condition of the weld itself. The integrity of the weld surface is crucial for effective liquid penetrant testing, making the potential for smearing a significant concern in ensuring accurate results.