I did do it once with filler but the rust came back after 5 months so the best way of getting rid of it without replacing the whole wing is cutting that section out where it's rusty and welding a piece of metal into the exact shape then spraying it maybe half of the wing so that it blends in depending on the condition of the paint on the wing it might cost you no more than £300 I think but at least that rust won't come back again and the finish was perfect you couldn't even notice here’s a few pictures of mine when it was done last year