I have sprayed the frames of my pickups (three Chevys and now a Nissan) since 1988 with WD-40. I never had any real rust on the frames or cross members even though I lived in PA and drove on heavily salted roads. Now keep in mind my frames had a coating of scmutz on them that was factory applied and the WD-40 soaked into this material. On a bare painted frame I doubt it would do much at all.
On bare painted metal like the control arms, I always applied a coating of grease when I lubed the ball joints. Since my Nissan has sealed joints, I have not been as regular about this and at 7 years, I notice some surface rust forming on the lower surfaces.