NAPA sell this rust eliminator stuff... go buy a bottle of it (about $9, but it works great). sand the area down as smooth as you can get it (if its a hidden area, dont worry about how smooth it is), just get the paint and as much rust off as you can.
after that, shake the bottle up, pour some in a cup, and paint it on with a brush. let it dry, then give it another coat for uniformity. the rusted areas will turn black, anything that isnt rusted will be clear. after the 2nd coat is dry, prime it, seal it and paint it. dont, for the love of god, sand the rust eliminator. you can spray a couple coats of primer and sand that, but leave the eliminator as is.
ive used this stuff all over on my supra, and ill tell you, after a couple other
ups, and having to sand it back down, and seeing no rust, ill live by this stuff.
do you have an undercoating on your car? if not, id recommend it. itll stop any more rust, and it protects your floorboards and stuff from rocks and other things. you can buy 3m rubberised undercoating for not too much, just wrap everything you dont want sprayed and go to town.
speaking of focuses and rust, does anyone else have rust around their gas filler cap?
and dont use wd-40... use something thats made for rust treatment. if youre going to try and paint over the area, wd40 will
up the new paint (just the chemicals sticking to the metal, no matter how much you clean it).