What is wet fluorescent Magnetic Particle test? Is ASME Sec VIII has asked to do this for LPG staorage tanks?

Wet fluorescent Magnetic Particle test is done in cirferential weld joints including heat affected zones from inside: 100% "T" weld joints from inside, 00% weld joints for nozzles, 100% support columns - pad plate to shell: 100%. Why this is done in an LPG sphere? Is their any other method also to do this?