We fix the source and not the symptoms.  Moisture can settle inside or behind your vehicles body panels. Today most car manufactures use a "foam like" adhesive along or between body panels to reduce road noise and vibration.  These areas can also trap moisture and normally have less paint for protection.  Eventually causing moisture to penetrate down to the metal.  Rust will form and spread quickly.  Its like a cancer to certain metals.   We cut it out like a surgeon would who is operating on a patient.   We do the repair by replacing or repairing the effected zone.  Then treat the area with Etching or rust preventative substrate.  Next we seal the area with Epoxy primer to protect from future exposure.   Finally, we finish off with an appropriate color to match.