iv got a spot of rust on my rear left quarter panal, right above the wheel. its almost eaten through the metal flange, and its about to creep around the corner so itd be visiable when looking at the side of the car. is there anything i can do to stop it?
You need to take the car to a competent body shop and have them sandblast all rusted material away and weld in NEW metal and use POR 15 inside. A small patch will work and try to seal the seam where water got in in the first place. Don't let them just bondo it up or cave and pave with duraglass (beat it in and fill with duraglass or bondo) that won't last long. The metal termites seem to like Mustangs.