I lean towards there always been some sort of resistance, whether it is a finesse roll, magic resistance, or the target rolling athletics to run from a falling boulder. Thematically, that's the way I feel it should be, as a storyguide and when I'm playing characters. To me, this is mostly an intellectual exercise; In practice, spells either need to penetrate, or finesse, or are avoidable by the target's skills, or possibly a combination of all of the above.
A zombie wielding a sword, a magic horse with mundane horseshoes, an animate statue wearing silk gloves. At what point does the magic need to penetrate? (My friend ruled on that last one that the non-penetrating golem hit about as hard as someone throwing their silk glove in your face)
I think that's probably right. Although I like the mental image of a necromancer pouring a bucket of paint on a zombie.
Parma blasting charged item paint using the tCI guidelines? Guaranteed colour-fast until the magic goes off. 
I imagine the necromancer making them walk single-file through a paint-spraying machine, like in a car factory. After all, the best way to get a truly vast zombie horde is to use mechanisation.
that's how I became a zombie