All of our materials are compatible but if you want to keep Liquitex Acrylic Gouache’s formula as matte and opaque as intended, we would recommend using a Liquitex gel medium rather than a fluid medium. The gels have an optimum consistency and Liquitex Ultra Matte Gel is particularly good to use in combination, as it effectively maintains the matte surface shine. Liquitex Light Modeling Paste and Flexible Modeling Paste are also good partners for Acrylic Gouache, while Modeling Paste can be more prone to cracking due to the paint’s high pigment level. From the Effects mediums, String Gel and Pouring Medium are recommended.