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