Things You'll Need
Flat headed paintbrush
While acrylic paint is very sturdy, after it dries on a glass surface it can chip, peel and flake off quite easily. Acrylic varnish is most often used to seal paintings and protect the surface from damage and fading, but it also works very well when used to prevent acrylic paint from flaking off a glass surface. The varnish is available in art stores and online is simple and effective to use.
Dip a flat headed paintbrush into some acrylic varnish. You can choose to use a glossy varnish, which will dry shiny, or matte varnish which will dry without shine.
Glide the brush across the painted area on the glass, from left to right. Continue painting it on over the glass in the same direction in a thin but even layer.
Allow the varnish to dry completely.