Conductive body of flashlight
You are talking about DC, so there is really no skin effect or the like. The whole body of the flashlight is conducting the DC, the external varnish is only for corrosion resistance and "look and feel".

If you scraped off the coating there won't be dangers for the user holding the flashlight. The voltage drop along the case is ridiculously low since the resistance is probably on the milliohm range and the internal batteries+circuitry (even assuming a boost converter of some sort) won't generate more than a few volts maximum.1

Moreover the body of the flashlight is probably some aluminium alloy, already fairly resistant to corrosion, and some models are made with no protective coating at all, so the conductive surface is exposed to the user.

1
Ok, theoretically I could imagine some idiot el-cheapo manufacturer using a string of some 40 LED in series and using a ~150V output boost converter to power the string, but I still have to see that :-)