You need to allow adequate 'dry-time' before you drag your hand across the paper, or you'll spoil that birthday-card or greeting card or letter on specialty

Damp wet hands, rain, sweat, condensation from a drink... any amount of liquid moisture smudges the ink.

I am left handed and frequently smear ink due the position of my hand.

However they can smear on paper if you smudge your hand after you write something

They scratch as you write and fade in and out.