Why not replace the fenders? Even good clean used ones will do. If you are intent on repairing, cut back the metal until you have no rust showing. grind off the paint several inches from the cut line. If there's an area thats still rusted, more than surface rust, ie ; a grinder wont remove it. Rent a sandblaster that has a vacuum attatched. Its the best way to remove rust which will be pitted in the metal. Dont trust inhibitors, they only work on light surface rust and will not get it all out. If you dont want to or cant find a sand blaster you can work on the pitting with a small pointed die grinder tip.
As for welding the metal, do you have any experience sheet metal welding? If not, consider getting someone who is skilled to do it for you. Unless you just want to spot weld a patch on. You will get quite a bit of heat distortion. The trick is to shrink the metal after, which has expanded. Its a bit of a lost art form in these days of "bondo everything" and generally poor workmanship in the industry. If you are really into it, I could describe this process to you.
After you're done sand blast all the welded areas, so there is no discoloration left, it looks all white and all the pits and imperfections in your welds are clean. Then prime both sides, I like to use rustoleum red oxide. You can use the filler of your choice on the outside, make it straight, no deep scratches , then prime, final finer grade sanding, then youre ready for paint. On the inside , use a seam sealer on the welded areas, then undercoat with a thick rockguard. If its in an internal area you can spray wax-oil in there. This will last indefinetly!
