i had the same problem on my 90 accord, as well as nearly any older honda ive seen they all have rust in the rear fenders...
Your gonna need to cut out the rusted section, Cut out some sheet metal, weld it in, Bondo over it, sand it and everything
ITs gonna be tough because theres a contour right at the lip of the fender you would have to bend the sheetmetal to the same contour...