About "Dark Mask"

By Rese, in Dark Heresy

There is a coating "Dark Mask" from TLD book

Throughout the Calixian underworld there is a rare substance commonly referred to as Dark Mask or Lacuna dust, although it is not known if Lacuna is its true world of origin. Said to be a darker than the void itself, the substance can be used to cover a surface, or even a person, rendering him all but invisible in low-light situations. Even active scanners have difficulty locating items coated in Dark Mask. Dark Mask can be applied to anything, including weapons, clothing, and bare skin. When hiding in shadow or during low-light situations, Dark Mask gives a +60 bonus to Concealment Tests. Characters using scanning equipment, such as auspexes, can only locate someone or something covered in Dark Mask if they pass a Very Hard (–30) Tech-Use Test when using the device. A single canister of Dark Mask contains enough dust to cover a single standard- sized human (plus clothing and basic equipment)

What do you think, does it come off ? how long it works?

If it's dust as the name "Lacuna dust" implies, I'd assume it comes off with time (assuming you're moving around, sweat will probably make it run on your skin, and it would probably shake loose of clothes). A shower or similar will probably remove it from skin (a washing for clothes), and wind or rain (weather in general, really) would probably damage the coating too.

Likewise, touching an item that has been covered in dust will probably disturb the coating, and handling it regularly (as you might a weapon) will probably remove it fairly quickly...