91 Civic with same rear fender rust problem here.

Don't laugh, but last winter I cleaned up the rust on the fenders as best I
could (steel wool, paint thinner) and applied roof cement. (That's exactly what
the tube says on it, in case you're not familiar with this roofing caulk
material.) My Civic is black, and it did look better than the rust for several
months. Now it's fading in color.

At Pep Boys, I saw kits that had some kind of plastic in them that seemed to be
for major "fill-in" jobs like this. I'll probably try one of these next. Also,
I've been working on my car a lot in the last few months and see that it may pay
to take off the rear wheels and go in through the rear wheelwells next time.