Almost any petroleum based solvent will work.

I keep a can of charcoal lighter fluid in my chemicals. It's cheap, less likely to attack any plastics or finishes and stays "wet" longer than mek.

If charcoal lighter fluid won't work, then I use something a little "hotter" like lacquer thinner, and then acetone, and so on.

Keep the charcoal lighter fluid handy when covering... while mek or acetone will clean the smudged backing off the iron, the charcoal lighter fluid won't evaporate off the hot surface so quickly. Furthermore, using mek or acetone will remove some of the coating off the iron surface in time whereas the charcoal lighter fluid won't.