Why couldn't he simply turn into fire and repair the damage?
You can read here that:

Users of Busoshoku Haki can strike Logia users as if they were human; Rayleigh describes it as the ability to &quot;force into solidity the fluid bodies of the near-invincible Logia Fruit users&quot; when he explains it. When Haki is applied to a Logia user while they are intangible, rather than nullifying the powers completely; it turns the user back to their physical form. It also allows the one using Haki to grab the person in their transformed state as if they were tangible.

From the manga:


Chapter 597 Page 18.