I know your post is pretty old, but im currently treating rust on my 96 sportrak, thats spent most of its life by the seaside.
The rear of the vechile suffers the most, above the petrol tank, behind the fuel line cover, rear wheel arches, pay attention to areas with metal plates on top of others, it may look ok, but the crappy plasticly coating can habour lots of rust cells. Which you dont know about untill you poke it. Other than that is the sills.
If you pull up the carpet in the rear have a good look around, any rust you see on the inside will indicate a problem on the opposite side.
As for the petrol tank, im going to make a deflector plate to stop mud getting up there, there was tons it cloggs up drainways, absorbs water and sits there slowly rotting your car away