Washed car the other day and found small area of rust at bottom rear of front wheel arch / sill junction. Probed the drain hole underneath and seemed partially blocked with earth. Intrigued I undid bottom securing screws for wheel arch liner and probed behind with screwdriver. Came out covered in earth. Then jacked up car took wheel off and removed wheel arch liner. Bottom part of wheel arch and inner wing were crammed with wet earth. Proceeded to remove many handfuls of earth from area (couldn't believe how much had collected), wash it all out, clean it up, treat rust, repaint and refit. Half hour job turned into half day but glad I found root cause! I take care to wash out wheel arch when I clean car but obviously each time some earth gets behind liner and can't drain away and gradually builds up. Worth having a probe behind liner to check yours aren't same as perfect conditions for rust to start. Boring but true ...
