The only ways I've seen it done is:
• picture, upload, print, decal
• mask, airbrush
• sable brush with hobby paint
and this is in reference to HO and N scale at to scale
Rustofills, it also depends on the era of the layout you saw. Some model RXR heads insist that graffiti on their trains are not allowed based on time era of the layout that they are building. Then others with modern layouts just refuse to include it because it's illegal, eye sore etc..
I've discussed this with a local RXR club and they won't have it
Zed, thanks for looking in and the comment on the BNSF project. I'll post the test paint practice there, GWRXR, and materials used. I haven't started the L&N yet. Your hopper conversion and weathering is outrageous real looking. Keep it up