When a 'simple' aging process went sideways on set

Trying to age a brand new, pristine leather messenger bag 20 years in less than an hour taught me a harsh lesson about material reactions. I figured a basic spray of watered-down acrylic paint (burnt umber and black), a bit of sanding, and some scuffing with steel wool would do the trick.

The acrylic worked okay for the first layer of grime, but as soon as I introduced the steel wool, it started stripping off the paint unevenly, exposing the pristine leather underneath in blotchy patches, rather than subtly degrading it. Then, attempting to re-apply the paint resulted in a sticky, inconsistent mess that looked more like clumsy painting than genuine age. What worked, eventually, was ditching the paint for a careful application of dark shoe polish and a much finer grit sandpaper (220, then 400), followed by some actual dirt and coffee grounds rubbed in. The shoe polish stained the leather itself, creating a far more natural, built-up patina that the acrylic just couldn't achieve.

It solidified that aging isn't always adding; sometimes it's about sympathetic degradation. What materials do you find consistently problematic for rapid aging on set?