Liquid penetrant inspection
Procedure used for surface inspection of hull welds. A liquid penetrant, which may be a visible red liquid or a fluorescent yellow-green liquid, is applied evenly over the surface being examined and allowed to enter open discontinuities. After a suitable dwell time (minimum 5 minutes), the excess surface penetrant is removed. A white developer is applied to draw the entrapped penetrant out of the discontinuity and stain the developer.
Further reading: ABS Guide for “Nondestructive Inspection of Hull Welds” (2002), can be downloaded from www.eagle.org