I don't know if PAM would work, because I've never tried it. You could experiment with it. I have only tried with vegetable oil. You could try using baby oil or something like that. Do you not have vegetable oil available?
Seriously though, having something like oil between the background and glass really enhances the background. I first noticed this when i was changing water at my cousins house on his 10 gallon tank. He had one of the usual plastic store bought backgrounds with the pictures on them. When some of the water got on the background, the background seemed to fuse with the glass and the picture became bright and lively. In the places where there was no water, the background was dull and hardly any light was hitting it. You could hardly see the background when there wasn't any water on it.
The oil acts like the water did, but it doesn't dry up as fast as the water does. Try it with water and see what i mean.
Seriously though, having something like oil between the background and glass really enhances the background. I first noticed this when i was changing water at my cousins house on his 10 gallon tank. He had one of the usual plastic store bought backgrounds with the pictures on them. When some of the water got on the background, the background seemed to fuse with the glass and the picture became bright and lively. In the places where there was no water, the background was dull and hardly any light was hitting it. You could hardly see the background when there wasn't any water on it.
The oil acts like the water did, but it doesn't dry up as fast as the water does. Try it with water and see what i mean.