For best results, please make sure that UV coverage is less than 30%. Otherwise, the cards will be stuck together and become a block of cards. Also, The mask file must be vector art created in a vector-based program such as Illustrator. Otherwise, you won't have sharp edges on the artwork. Up to 70% coverage is acceptable only with patterns, thin strokes, or small shapes. Ideally, the size of the area, shape, or image covered with raised foil should not exceed 1in x 1in. Here is an example of a Black Mask file used in a Spot UV application:
​