You may have to go more drastic Steve. It depends if it's bleached by the sun, or if it's a residue from hard water. Meguiars do an ultimate cutting compound and a milder finishing polish. Look at their product range, and decide which you fancy. Red is quite prone to this, and I've already had problems with my Golf when I don't leather it off after washing........

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4