Pick a rust spot on the car that you want to clean. Thoroughly wash the area around the rust spot with water. This will wash away any dirt or grease that may pose problems later on. If the grease does not go away with water alone, use some soap to clean off the area and it should do the trick.
Using a piece of dry cloth, dry the area properly from which the rust needs to be removed.
Fragments of rust may spoil the painted area of your car. Also, you will be doing vigorous sanding in the steps to come. So, using masking tape, tape on the area surrounding the rust spot.
Image curtesy: redlandstruckservice.com
Now start rubbing the rust spot using a strip of sandpaper. The idea is to push the sand paper with only the index finger because it applies optimum pressure in circular form to remove the rust. Keep on sanding the rust spot until all the rust is completely gone.
After removing the rust, blow away any excess. Then wipe off the area with a damp cloth.
When all of the rust is gone, give the spot a close look. In case you still see any rust remaining, repeat the sanding and wiping steps again until you see the metal clearly with no rust on it.
Remove the masking tape and once again wipe the surface with a damp cloth.
Apply a uniform layer of primer on the sanded area. Make sure that you use the primer because if you leave the metal bare, rust will reappear once again in a few weeks at the most.
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If you have anymore rusty spots on the car, repeat the above mentioned steps again. Or once the primer is completely dry, you can paint that area. Once the paint work is complete, your car has one less rusty spot.