When you paint a rusty piece of metal you can trap some moisture and oxygen under the paint and the rusting process will continue until one of the three required elements (iron, water, oxygen)are used up and then the reaction stops. If the paint seal stays intact the rust is still there, but the rusting process has stopped. If the seal is broken (Paint Chips off)the process will begin again. Remember that to effectly protect the metal from rusting you need to seal both sides of the metal. Painting just one surface leaves the back door open to rust (Bubbled paint w/ rust underneath). Remember also that two of the required elements (water & Oxygen) are present in the air, so the rusting process can continue on our cars parked in our garages. That is why cars last so long w/o rusting in really dry climates like Arizona due to low humidity, we all know they have plenty of oxygen in AZ.
