Some time ago we came across a subtle magnetic nail polish. It had fine magnetic dust in it, and could record the local magnetic field profile at the time that the polish dried.
But hey, why can’t you do this with full-on iron filings? So, for our own bold and impractical take on this concept, we tried mixing genuine iron filings with nail polish (clear, in this case). Mix well, paint on, hold finger over (large) magnet while it dries. Don’t even think about trying to fit those spiky fingertips into gloves.
Results? So-so. The particles aligned with the field and solidified, but have more clumping than we’d like to see.
Maybe slightly finer particles would have been better. Much better would be if we found a good way to work with ferrofluid that could be hardened, or perhaps a version of magnetic viewing film that could be painted onto surfaces. Or maybe, if our version above were redone with RTV silicone, the particles could wiggle around in the presence of an external field.
We leave these important questions to higher minds than our own.