Boroscope it or walk. Outside has clear surface rust that’s been spray painted as you already mentioned but no telling what the inside of the frame is doing.

Check the front auxiliary crossmember where the front skid mounts, that rots pretty damn quick/often. Check the section of frame near the front cab mounts. Check the area near the rear lower arms (rots out) and also check the rear crossmember.

By check I mean whack it with a hammer to see if you get a nice ting or hollow thud, and screwdriver poke it if it’s suspect.

Otherwise hard pass on a rust belt vehicle seeing as your in Cali. You should be able to find a better rust free option where you are at.