You might want to research the work of Mexican muralist, Diego Rivera for
ideas. Also a commercial paint store would probably be the best source to
get advice about the surface you are painting on--it probably will need to be
primed. I also understand that household paints such as Sherwin Williams
is the best to use for wall murals. Perhaps a local paint store would donate
supplies for such a project!!!!! Worth a try!!! I would also suggest that
you compose your design on a small scale (18" by 24"), grid it off in 2"
squares and transfer it to the wall using larger squares according to how
large you intend for the final mural to be.
I have just been asked by our coaching staff to paint a mural of a rock wall
in the gym (a climbing wall is going to be installed there) and these are the
steps we have been advised to use by a professional artist friend.
Good luck and let me know if you hear of other ways of doing a mural.