it's a design flaw from honda... even the best kept 4th gens in snowy regions start to get this if they were used as winter cars. keep in mind that if you decide to bondo it, you have to remove as much of the "infected" metal as possible, and then use a rust killer on the part that remains. i find the stuff that turns rust inert and intoa primer works best.

you can domo arigato to the honda engineers for that beautiful brown patch we all know and love.