When Iv had my roof down and then raised it , Iv notice dust marks on it , a good clean of the parcel shelf cured that issue. A few weeks back all the cars in the sorounding area were covered in a fine yellow dust and the paintwork was rough and sticky, turned out to pollen from the beach trees, may be as Dr Nick suggests ,tree sap , why is in a band across the roof ??? ,'stuff' may be being deposite onto the parcel shelf when the roof in the open position ,