in changing my door out i found theres a hiden rust problem, mind you im in canada where we use alot of salt ..anyways on the doors (mines a coupe not sure about 4 dr) the weather striping on the door closest to the outside(sorry dont know the name) follow it down to the point where it ends by the hinges about an inch up there is a plastic pin that goes into the door to hold the striping in place , there seems to be a problem there with water sitting or something but my car had considerable rust for a 5 year old car , so check yours out!! let me know if you find the same thing..

 

Hopefully, you (or the original owner) weren't taken in by the Rustproofing/undercoat after-sales which were pushed for years.
One of the paradoxes of having a car rustproofed, is that if the job is done too well (excessive material sprayed) the water drains which are purposely designed into the bottom of doors are clogged with dried rustproofing materials. Thus trapping water in the bottom of the door (and it WILL find a way out, usually by corrosion)
An other issue can be the sand blasting of the outside paint. This occurs from salt and road debris being thrown up by the front wheels and impacting against any exposed edges (lower front edge of the doors, front edge of the rear quarter at the door opening. Mud flaps control this
i hope you weren't a victim of either of these. My entire stable of Honda's (except the '88) are rust free. And Rhode Island uses salt with the same enthusiasm as the best of them.

P

 

sand it off, prime it, fill it, paint it, and clear coat it.