Hello, Leili! I use Golden Archival Varnish spray......one can use it on any medium, including watercolor. Since I paint ACEOs and small works of art, this Spray protects the painting from handling, humidity, etc. (since many folks who buy ACEOs do not put them under glass). They have Gloss, Mat and Satin....I prefer the Satin for my watercolors, but I think Mat may be best for Gouache. One can control the spray: the lightest of sprays to firmly coating the painting through successive light sprays, letting each dry in between. Best used outdoors or in a well ventilated area. I use the covered part under my stairwell...I also keep the painting there until the spray dries (stinky stuff!)...works really well, and keeps my home free from harmful vapors!