I LOVE to paint on up-cycled items. Nothing better than taking something that may go in the trash and making it brand new. I've painted windows for many years, thanks to my BIL's home remodeling!
The picture above is one I recently painted and is for sale HERE.
You'll notice that I left the background clear, which can be a challenge to varnish. Since I painted this one on the FRONT of the glass, I went over the entire painted area with varnish, extending the varnish just slightly onto the glass. You never want to varnish across the entire front if clear glass is showing. You will see brush marks on the clear portion.
This window was done a different way:
I reverse painted the design on the BACK of the glass.
This takes a bit more time and effort as you have to work in reverse. Since the painting is behind the glass, you can leave it unvarnished. A great way to do this design as it would have been difficult to varnish those individual pine needles.
And finally, this window was done by painting ALL of the front of the glass:
Once the painting was done, I simply varnished the entire piece. You'll also notice that on this piece, I painted and crackled the frame. This picture hangs in my parents half bath, a perfect place as that room doesn't have a window!