Alex Sumner answered:
OK speaking as a ceremonial magician here… the rationale behind the Pentacles of Mercury or indeed any other planet, from the Key of Solomon, is that when you uncover them, you attract Spirits towards you, even causing them to manifest: and when you cover the Pentacles up, you send the spirits away.
My first thought on seeing this question was “Mercurial spirits might be cool to evoke every once in a while, but do I want to evoke them all the time? When it would be inconvenient for me? To the exclusion of other types of spirits?”
Let’s face it, taking this literally, you could not get naked without being interrupted by a Mercurial spirit. Taking shower or having sex would become a nightmare! Conversely, if you were at work, and you needed the assistance of a Mercurial spirit to help you make an important business decision, you would have to strip off in the middle of the office. If you were at school or college, and you wanted a Mercurial spirit to help you with whatever you were working on, you would have to get naked in the middle of the classroom or lecture hall.
More seriously, there is a school of thought that says that by getting a tattoo of a spirit, you get owned by that spirit, which is not the kind of pact you might be wanting to achieve – i.e. better to establish a healthy relationship with the spirit by evoking it in the proper manner.
So tattoos may be generally cool, but I’d personally think twice about getting one in the form a pentacle from the Key of Solomon.
Alex Sumner’s answer to Would it have any harm if I get The Pentacles of Mercury tattooed on me? – Quora.com