What material is the fence? Is it a wrought iron fence, a wood fence with rust stains from hinges....?For a wood fence, I would sand the stained area and wash w/ ammonia/water mixture. I would then prime the area with Kilz and repaint.If it is a metal fence, make sure the rust isn't actually cancer. Cancer is where the fence is rusting from the inside out. If this is the case, want to or not, you're getting a new fence. Or at least replacing the rusted sections. If it is not cancer, scrub it with an steel brush, then sand with a medium grit sandpaper, then prime with Kilz, then paint with the paint of your choice. The steel brushing and sanding will remove the surface rust, and provided that's all you have the repainting will seal and help deter future rusting.