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Rust is usually placed into one of three categories based on how serious it is, but penetrating rust is the worst. Check the rust spot for signs that it has created holes through the metal or that the metal has rusted all the way through. If it has, that metal can’t be fixed; it must be replaced.  Surface rust is light, and is the first sign of rusting. It is surface deep and usually forms in scratches or nicks in your paint. It will just look like a bit of rust on the metal. Scale rust is more serious, and it develops after surface rust is allowed to spread. It’s the most serious rust you can address easily at home. It may include some paint bubbling or flaking of rusty metal. Penetrating rust develops after rust goes untreated for a long time. If there are holes in the metal or the rust goes all the way through, the only way to fix it is to cut the affected metal out and weld a new piece in its place. You will need to repaint the part of your car you remove the rust from, and that means needing to find an exact match for your vehicle’s paint color. Most cars have labels on the body inside the driver’s side door frame and occasionally under the hood that will list that “paint code” used by the manufacturer. Give that code to the clerk at the auto parts store to get a can of paint that matches it exactly. If you can’t find the paint code on the body of the vehicle, you can often find it on the manufacturer’s website or occasionally in the owner’s manual. You’ll need an automotive primer and paint matched to the vehicle’s color by using the paint code. You will also need a can of automotive clear coat. You can get these paints in spray cans, though you can also get it for use in paint guns with air compressors if you have one. For most minor rust spots, spray cans of paint are sufficient, but if you need to repaint an entire door, hood, or trunk lid, you may want to use an air compressor or secure the help of a bodywork technician. Use car wash soap, water, and a sponge to scrub any dirt or debris away from the area the rust is to make sure you are able to clearly define what needs to be repaired. When you’re done, rinse the area with clean water.  You may want to wash the entire vehicle to look for any other rust spots while you’re at it. Be careful scrubbing the rusty area, as metal flakes may poke into you as the rust comes up. You’re going to be sanding and painting the area with the rust, so you’ll need to cover anything you don’t want covered in sand and then sprayed with paint. Use painter’s tape to cover headlights, tail lights, windows, or anything else near the rust spot that you don’t want to repaint.  Painter’s tape will come back off of the car without leaving any adhesive residue. To tape off large things like a windshield, you can use plastic (like garbage bags) that you secure in place with painter’s tape.
Identify areas of surface and scale rust on the vehicle. Find the paint code for your vehicle. Purchase primer, base paint, and clear coat from the auto parts store. Wash the area around the rust spot. Tape off the body panel with the rust on it.