Assuming the previously painted window sills are clean, dry, sound and prepard as detailed in the technical data sheet we can recommend Coo-Var P101 Anti-Graffiti Coating.

This is suitable for application over water based paints once cleaned and lightly sanded, but we would recommend testing for compatibiltiy and adhesion prior to full application.

This would provide the best resistance to acidic sap and can be cleaned many times without effecting the coating.