I think I understand but my rendering program does not generate UV maps and I need a program that generates UV maps, Michael has listed some programs that can generate them but the demos do not allow the export of the map (you cannot save) so I couldn't test the program and the resulting map .... so I couldn't create anything.
Now I simply used a dirt map to simulate the dirty bodywork but it is not the same as a 3d painter in my opinion .... the effect is much more unreal than a dedicated map
about easy easy Paint or texture on 3D volumes take a look to the Paint 3D given (so free) by Windows (if you have this OS)
Not headaches and not long learning curves! :)
You can insert any volumes made by Moi ;) OBJ, STL, FBX...
