Thick nib Invisible ink marker PEN with high quality exterior grade invisible permanent UV ink. The ultimate in permanent ink. Nothing will get this stuff off. Choice of 2 colours that dry completely clear but fluoresce under uv either white blue or red. Totally invisible under ambient light.
Note that the red ink requires a UV light source of 365 to 380nm
The blue ink will fluoresce under any UV blacklight from 365 to 410nm
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