Lacquer is not an exterior product. It degrades quickly in sunlight and is too brittle for temperature changes. You are better off with a water-based acrylic or urethane finish. TJ Ronan makes a good clear finish for exterior murals. read more
I use a matte varnish for all of my acrylic paintings. It adds a nice finish to the painting, but doesn't have much affect on the color, as a gloss varnish does. I mainly varnish my paintings to protect them from dust and dirt, so I prefer matte varnish as I don't want my paintings to take on a 'gloss' affect, or any affect, for that matter. read more
(Glossy varnish would be too reflective for a wall mural, and probably look weird under interior lighting conditions.) Choose a varnish that is removable, in case you need to get underneath the varnish for conservation purposes. read more