Exterior murals require EXTERIOR paints to survive temperature changes, not to mention the sun’s UV and the crap that comes down. I feel so blue.
Below that, to make your work last, like canvas works of art, you can varnish.
Mural Paints:
- Brushing: use exterior house paint: Use the best quality paint you can afford, as always.
- Spray cans: I don’t know for sure, but I’m told by an artist friend that Montana Cans is the Banksy paint. It’s said to be known as the mural artist standard.
- Specialized “mural paint”: I have not tried these and my guess is that it is like any exterior paint, but in small bottles. The nice thing is that they give you lots of colors. Meh. If you get this, get Chroma Acrylic Mural Paint. Said to be the best.
UVLS: Ultra Violet Light Stabilizers
It protects like a coat of plastic, which is exactly what it is.
If you are varnishing over oil paints, don’t forget that you must coat the oil paint before it cures. Read about your paint and your varnish carefully.
Varnishing water-based paints does not have that time constraint.
There are two top-quality products for exterior (or interior) murals:
- MuralShield is an oil-based (mineral spirits/paint thinner to clean up). It comes in Matte and Gloss. One can can cover about 100 sq. ft. per coat for a smooth surface.
- Mural Guard is a water-based (clean up with water) when soft but you will need paint thinner after it sets. Apply with roller/brush or airless sprayer. Coverage: 250-300 square feet per gallon for textured surfaces.
I recommend the Mual Guard in the one-gallon size. It’s by far the best deal and you’ll have it for a long time. Plus, it’s not as toxic. Both are considered ‘the best’. 
Low-odor minearl spirits. IT says ODORLESS, but it’s not. But it IS good. I use it.