* Boiling Hair: When you apply the intense heat of a flat iron to wet hair, the water inside the hair shaft rapidly boils and turns into steam. This steam tries to escape, causing the hair shaft to swell and rupture.
* Damage to the Cuticle: The outer layer of the hair, the cuticle, is meant to protect the inner structure. The rapid expansion of steam can severely damage the cuticle, leading to raised, rough, and porous hair.
* Weakening of the Hair Structure: Over time, this type of damage weakens the hair's protein structure (keratin). This leads to breakage, split ends, frizz, and overall dryness.
* Uneven Styling: It's difficult to achieve a smooth, even style when flat ironing wet hair. The uneven drying and damage will result in a less than desirable look.
In short, you should always ensure your hair is completely dry before using a flat iron.