Homebrew PCBs

Hi all,

I obtained some surplus PCB material recently which was pre-coated 
with some sort of photo resist. The coating is red/orange coloured 
and in poor condition. My question is, How can I get this stuff off? 
I have tried acetone, alcohol, soap & water, citrus cleaner and 
Shellite (a hydrocarbon based solvent) all to no or little avail.

Anybody recognise this stuff and know how to remove it?

Thanks,

Morris