[quote=@thewizardguy]Why? This power isn't overpowered, so I don't see why you feel the need to inflict such a massive nerf.[/quote]

Here’s your own answer to your question.

[quote].... yes. He gains ALL of the properties of the material he turns into. So when he turns into steel, he acts like steel.[/quote]

Your artistic license is your character’s ability to change their substance at all.

[quote]I'd say he needs at least a marble-sized quantity. And yes, if he could acquire a marble of the material he could carry it around to transform. At which point he could indeed pull it out of his pocket and change whenever he feels like.
[/quote]

This is why this nerf is required. If you want the ability to change at will, whenever you want, requiring very little source material then that’s fine! If you want to be super durable, that is fine! But you cant be both. Characters that are min-maxed (like Natalie for example) dont get multiple powers or the ability to change their abilities at all. You can be the most durable character, a man of steel, but you cannot transform if that’s the case. You can transform regularly with little of your source material, but you cannot have full body transformation.

Unless you introduce another weakess that will balance having both.