Why couldn't he simply turn into fire and repair the damage? Spoiler alert, stop reading if you're not past One Piece episode 483.

When the guy (Akainu) put a hole in Ace, why couldn't Ace just turn into his element like all of those elemental Devil Fruit users? I get that magma was able to burn him, but how does that stop him from just dispersing like the other elemental fruit users?
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.