Replacing the fender and color match painting would be the cheapest option if you do all the work (even cheaper if you can repair the fender yourself). If you just want to fix the paint so the cracks don't get worse or let the elements get under the paint here's what I'd do... sand the area down with 1000 grit paper (this will help keep you from penetrating the clear coat) If you get red sanding residue stop sanding, the base coats are very thin on these cars. Once the areas are sanded you can clear coat the areas and re sand with 1000 grit paper between coats. After about 4 or 5 coats the cracks should be filled. Sand it again with 1000 grit then again with 2000 grit and buff it out.
