[...He could remove the stains, but he'd need to see the whole letter for that to make sure he doesn't remove any of the writing. So he doesn't offer.]

I don't need to see it; just hold it there.

[He lifts a hand, but not toward the letter; he summons his mirror to float beside him.]

Satillquinart Mullcreed.

[The mirror emits a brief glow, or more like a reflection of light, and then the letter also glows with a brief magical sheen before fading. Nothing seems too different, though the paper may feel slightly more sturdy than before. The mirror disappears as he lowers his hand.]

It's just a spell to make it more durable- it's not about to deflect bullets. But it should be better protected from wear and tear, and water won't dissolve it or wash away the ink. I wouldn't recommend letting it stay in water for longer than necessary, though.