Rocker panel rust

That is indeed structural, but uni-body construction is uncommonly strong. I can guess you never noticed anything wrong when driving the car. Most of the body's integrity comes from the floor pan and the roof. Even the glued-in rear window and windshield are structural components that add strength. There's a lot of cars on the road with this same hidden rust, and you never see one sitting on the side of the road, cracked in half.

There's not enough strength in the bottoms of the rocker boxes to make a difference related to injury in a crash. The bigger concern is when that rust reaches far enough, you're likely to see a flexing or shaking "B"-pillar, and moving doors. The "B"-pillar is what the rear door hinges bolt to.

If you plan on repairing this rust, be aware for the rust you see, there's two or three times as much that you can't see. If you cut rust away until you find nice shiny metal to weld to, you'll be cutting off a lot more than you expected to.

If you ask at any body shop, they will be able to give you the names of some suppliers they order replacement sheet metal panels from. For common rust repairs they see often, there's a good chance they will have a pre-fabricated panel that just drops in, but you'll need to see the new part first to know how far back to cut on the car.

Be aware too that where people can get into trouble is by actually making a repaired section too strong. For example, if you just pop a rocker box panel over what is left of the old one, without cutting all the old part off, that area can end up stronger than it was originally. In some cases that will defeat a designed-in crush zone that absorbs an impact slowly in a crash. When a structural part is too strong, that impact gets transferred somewhere else, and that is what could potentially lead to more injury. Those are the kinds of things lawyers and insurance investigators look for. They can even use that against you when it is their client who ran the red light and hit you. They'll argue you are partly to blame for your injuries because the safety system, (the body), was modified by you. That is the type of thing mechanics have to keep in mind all the time.

By the time that starts moving, you'll see the entire rocker box is broken free from the floor. With the doors closed, you'll be able to look down beside the seat and see the road.

The product you're referring to is something like "POR40", or "POR50". It's a rust converter that can be brushed or sprayed on. It turns rust into something else that looks black and gets pretty hard. My cousin used it on his Ranger and it held up real well for a couple of years until the transmission went out.