What would be the worst, most horrible thing to be gunged with? We've all done the usual with custard, slime etc- can people think of the most humiliating thing to have put down your boxers? Not saying I'd do it but the idea is tempting and a challenge is always good...
Similar to Spanknerd, i want to be covered head to toe in axle grease and thick smelly used motor oil while hanging from one of those engine lifts in a sling. another thing i would want to try is getting covered in the thick residue from the collection traps indoor paint ball places. its super hard to get off and can be kinda metallic looking which is a turn on.
another unusual item but its quite fun is nacho cheese sauce, its thick smooth and really a different experience.
I like Spanknerd's idea, too, but to add a wrinkle to it, I'd like to be mistaken for the mechanic by some unsatisfied customers, and suffer that fate. Add some pies to my face (what can I say, I'm a traditionalist!) from the break room as the coup d'grace...
may I recommend large tub of vaseline mixed with titanium dioxide (the super white sunblock lifeguards sometimes use). Feels cool, looks like cream, humiliatingly difficult to clean off (expect to feel lubed up for hours), and LOL assists in finishing off.