While creating outdoor murals, some of the issues that need to be considered...
Image - Cabbagetown
Info and Links to Information...
Selecting a Mural Paint - Liquitex Acrylics
Also, information about:
Soluvar Gloss Acrylic Varnish Liquitex
There is a great deal of information online from the manufacturers about the best use and longevity of their paints and products.
Varnishing an Outdoor Mural
Painting Exterior Murals
Other Links of Interest:
High quality UV protective varnish. Mix with water for several coats.
AP Fitzpatrick Fine Art Materials
Rustins Outdoor Clear Varnish - Satin
Flexterior - http://www.rustins.eu/ProdData/Flexterior.pdf
Yacht Varnish (Gloss)