I'm confused by the position that, something that is essentially transitory despite the duration (like a Thaumaturgy block), should be paid with by Refresh when a wizard has paid Refresh for Thaumaturgy. Disallowing a concept because of a GM's sense of balance is one thing, but saying to a wizard player "you can't do a thing within your bought Powers without buying more Powers" is...? How is that justified?
Well, where do you draw the line? By the novels, transforming into a werewolf is a spell, so Harry could do it. By the rules, you can use free invokes on aspects to pay for temporary powers. Push the power of that spell high enough to last a few human lifetimes, and you have a wizard with werewolf powers for ever. Do that a couple of times and he has all the powers in the book and more, while he has the same refresh rate as other characters. If you want something that is an integral part of the character (and a shield that lasts that long should count as such), just pay for it (by taking toughness for example).
And it is sort of an arms race that leads you nowhere. I generally apply the rule that whatever the players can do, NPCs can do as well. So why not have an opposing wizard who has a similarly prepared anti-shield-spell ready, and your shield goes poof anyway. Ok, you make a bigger shield, I make a bigger anti-shield, you make a bigger shield.... and so forth. Why not just get on with the actual wizards duel already? Get into the juicy bits.
And part of the rules is the narrative justification. Granted, that will vary drastically from one person to the next, but to me, a spell that powerful (I mean, we are talking about laughing at a grenade that is exploding in your hand) should have some narrative justification, something that it is linked to, something it can draw its power from for all that time, and so forth. Creating a spell like that, would be a quest all by itself, which should take a really long time. And even then, I would use this as a justification to take a power, not give a permanent shield.
Long story short: mechanically it is absolutely possible, yes. In terms of what I consider an exiting story to play out, not very much.
Players who want something like this are usually those who take all the defensive powers they can get anyway, in my experience. Make the character bulletproof, so to speak. However, in Fate I don't really see the necessity for this, because you can talk about the lethality and deal with it accordingly. Your player doesn't want his character to die from physical wounds? Done. There is no need to make him mechanically bulletproof.
