If you have a painted surface that is stained by rust, the first thing you need to determine is whether the rust is actually eating away at the metal under the paint or whether it is the result of another rusty item sitting on the painted surface and leaving a rust stain. If it is the latter, there are a variety of simple, green ways to remove the stain without damaging the paint. However, if the rust has worked its way into the painted surface, you may have to actually remove the paint in order to get rid of the rust.
Clean off the painted surface. Sometimes the stain will lift with just a damp cloth and three drops of mild liquid detergent. This will also enable you to view the stained area more clearly to determine whether the finish is part of the rusting process. If it is, then you will not be able to remove the rust from the paint but will have to remove the paint itself.
Mix up a rust-removal paste. In the bowl, combine a half-cup of baking soda, 1/4 cup white vinegar and 2 tbsp of lemon juice. If the mixture is runny, add more baking soda a tablespoon at a time until it forms a thick paste. Expect fizzing.
Apply the paste to the stain. You can let the paste sit on the rust stain for 15 minutes. It should not damage the paint itself, although the lemon may lighten the area slightly if it is in direct sun. Test this ahead of time if you are concerned.
Rub the paste--and the stain--off the paint. You will likely be able to see the rust "lifting" into the paste by the end of the 15 minutes. Use firm pressure and circular, polishing motions to rub the paste and the stain off the paint. If necessary, you can repeat the process four times.
Clean up the area. Use a damp cloth with two drops of liquid detergent to clean off the area and remove all residue from the cleaning agents. Then wipe the area dry.
If the rust is beneath the paint, you will have no choice but to sand it away and refinish the area.