All you need to know about Gouache auxiliaries
Gouache paintings can be varnished with Gouache Varnish based on non-yellowing synthetic resins. The glossy version 074 makes the colours slightly more transparent and darker, while the matt version 107 causes hardly any colour change. Both dry in a few hours, make the work waterproof and can be mixed together to a desired gloss level. Clean tools with white spirit. Another possibility is Talens Protecting spray 680, which is a quick-drying varnish in spray can based on non-yellowing acrylic resin. The more varnish is applied, the higher the gloss level and the more water-resistant the work. Protecting spray makes the colours slightly more transparent and darker.
Gouache is usually used on paper and the piece framed behind glass due to the fragility of the paper. A gouache painting then does not need to be varnished.