It will happen in an environment close to salt such as San Diego with the ocean air, or our Minnesota winter roadways. It usually starts at the edge of alumininum panels, or on light fixtures, wheels, exterior fittings. Our trailer had some when new (part of the bargaining) because of being towed on salty roads, or sitting on the dealers lot near the highway in winter.
The solution for us is to not try to remove it (you're just exposing more edges) but to treat it with Corrosion X or Boeshield, both available from the airstream store by mail order. I sprayed the edges the first time, but had a heck of a cleanup on my hands because there was way more product than needed all over. Now after washing the trailer, I use a Q-tip dipped in the product and wipe it on the edges where there is corrosion, or where I think corrosion may form. Next day or so I wipe it off, and it seems to be absorbed by the filiform keeping water out and preventing more corrosion. Whatever the reason, it has not advanced.
Also, I have sprayed the wheel wells, frame, and any underside steel components with Boeshield as protection.