When preparing metal flashing I was told by an ex painter not to scrape or sand these parts as it will take galvanizing off and rust,is this true?
Absolutely that is true. Two options for galvanized metal. If new, wash with vinegar to remove the oily coating which is part of the manufacturing process, then paint with a top quality acrylic house paint. If old, check your local paint dealer for a galvanized metal primer, apply and follow with an appropriate topcoat.