Why do you specify flexible? I use the rubberized asphalt coating they sell in spray cans in parts stores which sticks like crazy and lasts for ever. Cheap too! To prevent star bruised in the paint on fenders over the tires, glue in a strip of the butyl rubber roof sealer they sell at Lowe's or Home Depot. Thin, rubbery, made to stand up to the elements except for UV light but there isn't much of that under fenders, and again, cheap.