Try ironing on wax paper. My mom had me do this when I was doing some fusible applique at her house and accidentally tried ironing with the fusible side face up. Take a large sheet of wax paper and fold in half a few times, so it's either 4 or 8 layers thick, but still bigger than the face of the iron. Briefly iron the wax paper and then wipe iron on a clean towel. Repeat several times to get all the gunk off.