Spell Resistant Armor, with spray-on application Options

[Wolfgar]

Hello everyone, wanted to share a new item I'm house-ruling into my game. First, a little background. My players were recently hired to kidnap a young magician/graffiti artist who had developed a unique talent-- his street artwork was showing up in the astral plane, in perfect replication, and completely solid. Astral forms could not pass through his graffiti. The group was successful, and Johnson put the young magician to work. It's been a few months now, and Miss Johnson and her team have been able to produce the following item, in limited quantities. The players are probably going to be lucky enough to get their hands on some, if they don't screw it up.

Tactical Mana Barrier spray (Rating 1-6)-- This aerosol spray comes in nondescript cans, although dummy deodorant or cleaner cans can be used. Each can typically contains 3 applications of the spray. One application covers 4 points of Body (ex. a Troll with body 7 requires two applications). Once sprayed upon oneself (or on a drone, vehicle, anything that's not a barrier rules-wise) the user adds dice to any Resistance Tests vs spells, equal to the spray's rating. The spray's rating degrades overtime, at one point of rating per spell cast upon the subject (ex. said Troll has a fresh coat of Rating 4 TMB spray. After being zapped by a stunball, which he received a +4 bonus to resisting, his TMB spray has degraded to rating 3).
While under the effects of TMB spray, the user or object is highly visible in the astral plain, appearing as an anomaly similar to foci or true Mana Barrier spells.

I only plan on letting the players find a few cans of the spray. I have no idea what this item may cost, but it should be rather ridiculous to obtain. Any thoughts?