To turn joint compound into skim coat, you just need to mix in additional water to make it thinner. Joint compound either comes pre-mixed or as a powder you add water to and mix yourself. Skim coat is made of diluted joint compound.
A skim coat is also often used to even out and repair walls after damage or wallpaper removal. It can be applied with a trowel or knife and is then usually sanded for a smooth finish after two to three coats.
