I successfully stripped a Bachmann Spectrum shell using 91% isopropyl alcohol (not the more diluted 70%, or whatever, stuff). I'm sure it would also work for the Athearn shell as well.
I filled an empty glass Planter's peanut jar with alcohol, then submerged the loco shell in the alcohol. Make sure your shell will fit through the jar opening first - I had to turn and angle my shell a few times, but it finally made it inside the jar. Then just put the air/watertight cap on the jar and let it sit overnight. If the end of the shell sticks up above the level of the alcohol, try resting the jar on its side - sometimes you can get the whole shell submerged that way.
After about 24 hours, I took the shell out and scrubbed it clean with an old toothbrush. Worked like a charm!
Dan Stokes
My other car is a tunnel motor