Homebrew PCBs

I bought a lot of polyamide copper clad to experiment with, but the thing is that when i etch it with Ferric Chloride a shiny electrically resistive layer under the copper is revealed (im guessing it's the bonder or something like that). The only way I figured to get rid of it is sanding with a fine grit sandpaper and warm water after the etching, but the problem is that thin traces get cut or completely erased.

Do you know a better way to get rid of this shiny metallic looking stuff?