Glass is available in many forms and finishes and can vary greatly in texture, from slick window panes to cast glass with a texture similar to stone. We always recommend cleaning and testing as a first step.
Generally, Oils can be painted directly onto glass.
When painting with Acrylics, mix 3 parts GAC 200 and 1 part GAC 500 and add the mixture to paints until desired transparency is achieved.
Watercolors and Drawing Media can be utilized on top of an appropriate acrylic preparation like a pastel or watercolor ground.
Recommendations are for interior use only.