When dealing with a rust pit or even rusted out spots on farm equipment consider fiberglass or fiberglass based material. Fiberglass,fiber tech or even bondo glass will fix problems from rust pits all the way to rebuilding something. Why these materials? Simple they are easier to use than welding and it doesn't cost what a welding machine or welding materials cost. It also won't rust out with the bonus of being water proof and you can paint it. The only tools you need are good putty knives and an air sander with a interface pad. The pad lets you do edges and round surfaces easier than with the basic stiff pad. Another added bonus is that you can use cardboard as a mold or core to build with. If you brush or roll fiberglass resign it will stiffen up and you can build from that point. Use 80 grit sand paper to get it close to the thickness you want it then start stepping down to 220 grit paper. Always use a sandable primer when using any fiberglass materials so it gets a good even finish along with covering any pin holes you might have created while sanding down the fiberglass material.