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looks to be just surface rust.
Get a wirewheel and a cordless screwdriver and get to work.
Then go over it with an etching primer, then an epoxy then whatever color you want it, if you are looking to repair it that is.
It appears to be solid,I would give it a good solid poke to find out.Yes, there is a lot more concealed rust,if this is a car you are considerng buying,get it up in the air and have a very good look at the rest of it.
That looks like the end of the frame in front of the rear wheel. Look above that about 10" into the fender well, there is about a 2" x 4" "door" there that hides the rear frame mount. Remove the 4 screws, pry it off, and inspect that mount(or post a picture of it). If it's not any worse, you should be fine. It appears to me that car's been sitting outside a lot. As mentioned above poke around with a scewdriver.
You need to go over that frame with a hammer. Something like a pointed body hammer with a rounded point is good. You don't have to bang on the frame heavily, but if you don't hear a metallic ring when you tap and hear a thud/crunch instead that's a bad sign.
You can also reach into the access holes along the bottom of the frame rails and feel for thick pieces of scale. The differential crossmember seems especially prone to rotting and it is the piece that ties the front and rear frame sections together.
I would almost guarantee that if you tap around the area of that little hole on the left of your pic that you will find a more sizeable hole. Sorry to disagree with most of the other posters but I think the frame may have some problems.