Homebrew PCBs

Hi,

     I have some old PCB boards that were exposed but never eched. I 
need to find something to remove the old resist. I have tried Mineral 
Spirits, Lacquer Thinner, Acetone, Gasoline and nothing touches it. I 
would sure appreciate any ideas. I have not been able to locate a 
source for a "remover" though I know it is made. Also, is there a 
source in the US for Positive Resist in spray cans? I must be 
searching in the wrong place.

Thanks - Jack