When the surface is too smooth, the transferred image comes off too easily when you remove the paper backing. So, you need to roughen the surface a bit. With metals, you can use sandpaper or steel wool. For glass, I recommend
etchall to make a nice matte surface, like beach glass. We used etchall plus
ITS on one of our
hinged glass frame pendants for an interesting effect. I will link in a picture when I can track it down!