It depends on what you want to do with the truck. 87 F150s are known for rust issues around the rear leaf spring shackles; if the rust there is severe, you may be better off selling the truck, because rust in other areas will undoubtedly be bad. However, if the truck is only showing some surface rust, clean it, repaint the frame and grease it to prevent more rust. The 87 F150 is a great truck if well maintained.