Why should a part processed by visible dye penetrant inspection not normally be reprocessed by fluorescent penetrant?

The reasoning behind why a part processed by visible dye penetrant inspection should not normally be reprocessed by fluorescent penetrant relates to the interaction between the two types of dyes used in these tests. When a part is treated with visible dye penetrant, the residues from the visible dye can remain on the surface of the material even after cleaning. These residues can interfere with the fluorescent properties of a subsequent fluorescent penetrant application.

Fluorescent penetrants are designed to emit light under ultraviolet lighting conditions. However, the visible dye can essentially "mask" or diminish the effectiveness of the fluorescence, preventing proper inspection. The presence of visible dye residues can absorb or scatter the UV light, leading to a reduced contrast of the fluorescent indication, or it may even prevent the fluorescent dye from migrating into the defects being investigated.

Thus, once a part has been inspected with a visible dye penetrant, it is advisable to avoid reprocessing it with fluorescent penetrant to ensure accurate and effective inspections without interference. This is why understanding the compatibility of different penetrant types and their residues is critical in the liquid penetrant testing process.