Steel wool for tough stains
If you need to remove tree sap, adhesive residue or other tenacious stains from your windows, try #0000 (4 ought) steel wool. Lubricate the glass with a spritz of window cleaner (Windex) and buff away the stains with a wad of super-fine steel wool.
Bring out the power tools
Scrub away the toughest stains on glass with a small buffing wheel and a mild abrasive (Soft Scrub, Bar Keepers Friend or Bon Ami). When you're working near the window sash, protect it with masking tape.