I had the same problem in my 200SX
1. I used a die grinder to remove all of the rusted metal
2. I then sprayed the area down with rust inhibitor
3. Then did a bondo body filler coat to make the area look decent
4. Sanded the bondo
5. Primed and painted
Sanding/Grinding will get rid of most of the rust but it will also take little bits of it and grind it into the good metal and the surrouding area, making the problem worse.
What you really want to do is sandblast it because it gets rid of *ALL* the rust and it won't come back for a long ass time if you prime/paint it right away. But remember - 11 minutes after steel is exposed to oxygen it begins to oxidize so sand blast it and shoot it with rust inhibiter and primer ASAP. Also, make sure you paint it right after because primer is NOT a sealer and it will rust relatively quickly as well.
You can leave it with just primer on it for a week or so if you're not driving it, but I wouldn't recommend it.