You can protect outdoor murals a number of ways but the simplest involve preparing the wall and preparing the paint with treatments that you can purchase at most art shops. This can be done in as little as a couple of hours depending on the mural size. It is important to protect outdoor murals with specific varnishes as they will be exposed to weather that indoor art never experiences.
Prepare the wall before painting if possible. Examine the wall and determine the material. You can treat the wall with mild muriatic acid or even distilled vinegar. You can also repair wall cracks.
After the mural is painted, apply varnish to ensure that it lasts long and looks as bright and clear as possible. While the term "varnish" means any translucent coat that protects an item, and mural or art varnishes can be purchased at specialty shops which guard against the sun's ultraviolet rays as well as against rust and other hazardous events.
Once your mural is finished, you can obtain a permit to place signs instructing people not to touch or damage it.
Finally, avoid using water or water-based paint. Use acrylics and oil-based paint, as water often trickles down murals during the long painting process.