[DeTomaso] Cleaning Glass

[Will Demelo]

I've removed the rubber gaskets from the quarter windows and I'm having trouble cleaning the glass near where the gasket edge was. I've got the whole window clean except for this edge of 35 yr old scum line. 
I've bought an acid based window cleaner that doesn't even touch it.
I don't want to use anything too abrasive for fear of scratching the glass.
Any suggestions?
Will