Sub frame is off, stripped down, pressure washed and has light surface rust. A wire brush will take that off.
Are you restoring the car or cleaning things up for a great driver? On our (restoration) the sub-frame was media blasted and epoxy primed along with some minor rust pit repair on the right side (where battery is located). The media blast really assist with giving the paint grip and a good expoxy primer supplies the needed base. Also, probably typing in Eastwoods Chassis Rust Converter in the search icon will give some references to how it worked. I will only state since you have your sub-frame off, strive to do it the best possible way.