frame rust--how to fix?

I know absolutely nothing about body work, though I've used a sanding
disc and wire brushes before. I have rotary brushes for my drill, too.
I've got a bit of rust on the frame column between the front and rear
doors on the driver's side, around where the passenger door hinges
attach to the frame. Since this is practically the only rust on a '94
Civic I'd like to stop it before it gets any worse and before winter
gets cold here.

Once I take the door off I assume I just sand the rust down, hoping it
isn't so serious that I weaken the frame in doing so, but what should I
put on it to stop the rust I can't get out? I don't care about matching
the body color afterwards, as this area is visible only when the doors
are open. Is there an anti-rust primer, then I'd cover that with spray
paint?

I may also run down to the local body shop to see what they'd charge me
to do it, since I don't have a lot of time to deal with this before it
gets too cold.

Last car I had was a rusted hulk, so I kinda want to prevent the same
from happening here. It's a novel thing for me to be driving an unrusted
vehicle... <g>

Abe