The key to successful glass painting is to use brushes that are pliable. Soft acrylic or watercolor brushes fit the bill and work nicely when applying paint. Round, flat or angular brushes allow for interesting shapes, so keep that in mind when selecting the brush needed for a glass painting project. These brushes can be found in the fine art section of most craft stores and are relativley inexpensive.
Brush hair texture varies according to the intended use of the brush. Sable hair brushes tend to be soft and work well with acrylic, oil and watercolor paints. Stiff-haired brushes such as squirrel, or coarse synthetic brushes are better for heavy acrylic or oil painting. Feel the texture of the bristles when selecting a brush so you can tell if it's soft enough for glass painting. Medium-soft acrylic-haired brushes work beautifully for glass painting as long as they are soft, with a little spring to the brush hairs. Spring refers to the way the brush hairs spring back in place after a stroke has been placed on the surface. Some spring is needed. If the hairs are too soft (like those brushes used in ceramics) you may not get correct application of the paint where it's needed.
Using an acrylic brush, such as those offered by Loew-Cornell or Robert Simmons, will achieve the right application of paint to any glass surface. Loew-Cornell offers a paint brush line called American Painter; the brushes work well with all types of paint and have the right amount of balance in the spring of the brush hair. Robert Simmons brushes also work well with these various types of paint.
For large areas of paint coverage, use at least a #10 brush. The larger the size number, the larger the brush will be. Details are best applied using a smaller number, such as a 10/0 or #1 brush. The smaller numbered brushes allow for finer details such as lines, dots, dashes and swirls.
Angular brushes make interesting leaves, stems, lines and flowers. Flat brushes can be used for checkerboards or for filling in shapes. Round brushes lend themselves well to flower petals and round-shaped designs.
Identify brushes with research into what each is used for and how they react to the pressure of painting. Craft and fine art stores have specific sections for brushes, with explanations of what each brand can be used for and what paint best suits them. Long-handled brushes are usually used for canvas painting while shorter, easy-to-handle brushes work better for craft, glass and other surfaces.