This viscous substance will initially seem to be of a useful sort - acid resistance, elemental invulnerability, or slipperiness, for instance — until the wearer is under stress in a melee combat situation. 

At that point, he or she will have a 50% chance each round of fumbling and dropping whatever he or she holds - weapon, shield, spell components, and so forth. 

Only a thorough bath of solvent (alcohol, turpentine, etc.) will remove the substance before its potency wears off.