Even a tiny scratch can mar the appearance of any otherwise pristine porcelain enamel bathtub. Fortunately, repairing small blemishes and minor damage to your tub is not difficult, requiring a few products available at home improvement stores.
If your tub is marked by surface rust that hasn't penetrated the enamel or damaged the metal underneath, the fix may be as simple as using a cleaning agent to remove the rust staining. A mixture of salt and lemon juice applied to the rust and left to sit for a day will take care of many rust problems. More stubborn rusty areas may require treatment with diluted ammonia -- 2 tablespoons in 4 cups of water -- or a commercial rust-removal product. Bleach can make the problem worse, so you should never use it to remove rust from your tub.
Covering Chips and Scratches
You can cover minor chips and scratches in your tub's enamel surface with a touch-up glaze. Touch-up kits with glazes are made to match your tub's surface color. To repair scratches, clean the area around the scratch thoroughly with soap and water. When the area is dry, brush on the glaze and let it dry for 24 hours; repeat the application as necessary to fill the scratch. After the glaze is dry, use the kit's fine sandpaper to lightly sand the area, then finish the repair by buffing it with a soft cloth.
Deeper chips or cracks that penetrate the enamel and expose the underlying metal may require more substantial treatment with a repair kit that includes an epoxy resin. After you clean and dry the area around the crack, sand the area so the edges of the chip or crack are completely smooth. Prepare the epoxy and fill the crack, getting the surface of the repair as smooth as possible. After the epoxy sets, sand it smooth, then apply a finishing glaze to match the surface of your tub.
If the enamel surface of your tub is severely damaged or worn over a wide area, the best option may be a refinishing of the entire tub. Although tub refinishing is significantly cheaper than buying a new tub, it requires extensive surface preparation and the use of toxic chemicals, as well as specialized skills and experience; it's not a do-it-yourself project.