Wire brush back until you have bare shiny metal then treat with kurust or similar to stop the rust spreading. After that you need to asess how rotten they are and your game plan goes from there. To do it properly if you have rust holes you would cut back the rust until good metal and then weld in patches, then a light skim of filler over the top. If no holes then just a light skim of filler. Then primer then blend in the paint.