I don't know the specific product Magnasee, but have used a similar product in the past for audio head alignment. It is made by The Geneva Group, and is called MagView.
As I was using it only for viewing the track and not individual domains, I am not sure whether it is the same type of product, or what you will see under a high-power microscope. I am fairly sure that you can see no-amplitude to full-amplitude transitions... IF they are strong, well-defined and spaced out just a bit. It may also depend a little on your spraying technique. I only needed to "puff" my tapes, holding them vertically, to see the tracks themselves, and I had a graticule-slide to make measurements.
Since the iron particles are only magnetically stuck on the tape, careful wiping or washing of the card should remove it all with no ill effect. You don't want to run a card or tape past a magnetic playback head with anything magnetic and sheddable on it-- it is the tiny Gap in the head that can be corrupted with debris, and affect reading. But water and cotton balls will wash a tape or card off very well.
My can of Magview says: MAGVIEW PF-603, Geneva (c)1993, Minneapolis MN 55427-2848 USA, Toll-Free 1-800-358-5600. All it lists as ingredients are:
1,1, dichloro-1-flouroethane; iron powder; carbon dioxide.
I suspect you get your best viewing with a virgin card and a fresh recording.