2air has provided very good information. I would like to emphasize that the coating that filiform corrosion burrows under can be pigmented paint. In such a case, the corrosion is usually not noticable unless it is very extensive. Our Airstreams make such corrosion apparent because of the clearcoat. Not only is the corrosion visible, but its contrast with the surrounding non-corroded aluminum is very obvious.
The corrosion on non-coated aluminum parts is almost never if not never filiform.