I can understand your hesitation! Note that I removed much of the detail parts and handrail/grab irons, before cutting and filling.
This makes it much easier to mask for the grey band. I have to be patient when removing these details, as they are delicate and break easily. Don’t ask me how I know lol
Masking will be done by cutting an exact scale width of masking tape, according to Donald C Lewis’s book “Rail Canada Volume 3”
This is an excellent reference book for Canadian Modellers.
I have included a pic of the book
Al