Warding Cream
It’s kind of like shaving cream, only it’s supernaturally potent enough to be used to draw lines for pentacles and other kinds of wards. Warding Cream’s major advantage is also its major disadvantage: it works fine, as long as nothing physical touches it. But it’s very easy to disrupt it, and the foam will decay into useless no more than an hour after it is drawn.
Which is sometimes an advantage! If you’re doing a field scrying, and you don’t expect to have to deal with a physical attack, you can lay down a ward using Warding Cream in about one-third the time it takes to draw one normally. Also: if you have enough Warding Cream, you can temporarily make breaks in the lines and then touch them up with pretty much none of the usual hassle. And, best of all: when this stuff decays into uselessness, it also decays without a hint of a supernatural ‘aftertaste.’
So, as you can imagine: Warding Cream is a great thing, under the right circumstances — and from the point of view of espionage-style magery. It can even be disguised as a regular can of shaving cream. But don’t actually shave with it, please. This stuff is really, really expensive.
But how else am I supposed to charge my supernaturally sharp straight razor if not by shaving with warding cream?
Surely you already have a crystal-aluminum pyramid for that?
Have you considered a strop made of hardened white-stag leather? Has the supernatural pedigree, with a side benefit of Fortune if one follows certain Lewisean rites.