Ways to Increase the Difficulty of Boot Cleaning?

Has anyone got thoughts or tips on how to increase the difficulty of boot cleaning? I really enjoy having my boots licked clean, and occasionally a friend makes me do it. But what could be applied as a paste or spray to make the process more challenging yet enjoyable? Chocolate works well because licking removes some of it gradually, extending the time until they are fully clean. However, I’d prefer something with less flavor that still presents a challenge:mrgreen::whip: Any suggestions? Hot sauce wasn’t suitable since I can tolerate heat and spice. We need an alternative within the edible category that is safe to ingest. Does anyone have any ideas?