This trick requires: 1 hermetic magus, of average skill, who can enchant something with a level 7 perdo vim, in his lab. Assuming a standard hermetic lab in an aura 5 area (not uncommon) this isn't difficult.
Vis in enough of an amount to open a suit of full chain including quaff, and to enchant it with said level 7 perdo vim
Lab text for the following:
Perdo Vim 7 Enchantment
Strips 1 might from a demon that touches it.
Range Touch Duration Sun Target Individual Linked Trigger (Sunrise/sunset)
Said spell has NO penetration. As a result, by RAW, it skitters off the demon without touching it, as it cannot affect the demon.
By RAW (as I'm reading it, at least, correct me if I appear to be wrong) and ROP:I, a demon's weapon usually is a part of the demon itself, and thus touching it's weapon is touching it.
As a result, the demon goes to strike the armour, the spell triggers, fails to penetrate, but the demon cannot touch the armour for the same reason the armour cannot touch the demon.
Doesn't this allow any hermetic magus with a perdo vim total of 8 or so, even outside their lab, to enchant a suit of armour (might take a while with Perdo Vim 8, but it can be done) that allows them to not be touched by demons?
I'd argue that the demon can still strike the armor. Recall the rules of magic resistance in the core book: a magus falling on magical spikes takes falling damage, but isn't impaled. I would say that the demon hits the armor and can cause a severe bruise, but might/might not actually puncture the armor. It's the same way that getting shot while wearing a bullet proof vest can still hurt a good deal. Maybe an additional bonus to soak?
Also, work out in advance how your troupe will treat the pink dot loophole.
An interesting approach, but it still means you can use the pink spot trick to make your armour tougher against magic beings, demons and faeries. And probably angels too, but I'd rather not be testing to find out with that particular one...
The effect you present does not work the way you describe it.
The effect you designed affects one target that it is touching at sunrise and sunset respectively, lowering its might by 1 (if it even has any) for Sun duration, since the effect is only triggered twice per day at these specific times.
The effect you intend would require sensory magic as described in the Bjornaer chapter of Houses of Hermes: Mystery Cults. These effects cannot be enchanted into items however.
Going further, a magus that fast casts a non-penetrating spell against a demon is no safer from harm than a chainmail basically doing the same thing would be.
Or, looking at it differently, a magus with an ongoing low penetration shapechanging spell doesn't somehow repel demons because their Magic Resistance prevents them from touching him.