Just take your time with whatever needs to be done. After Dremeling off the fluting I sanded, filed, polished, buffed, whatever it took to get the roof smooth, went thru many sanding drums on the Dremel. I don't know if a mini-mill would be worth trying, for one thing you'd have to make sure the shell is held securely in place with no chattering of the metal as you remove the fluting. A drum on the Dremel was easy to guide and handle.
Good luck!!! If you have questions feel free, I just hope I can remember what I did so I can help.
