There are grades of paint, some of which are formulated specifically for
outdoor mural application.  The preparation of the wall is vital to the
longevity of the mural, this includes the material the wall is made from
and the priming that is done.  After the mural is finished, there are a
number of different types of coating that slow down deterioration and
inhibit the permeation of the mural from graffitti. I have overseen the
creation of six murals of the scale you are describing as museum-sponsored
community art projects.  I can put you in touch with consultant/muralists
who might be of help.

Paul Apodaca