If there is any layered rust get it off with a chisel or grinder (flap disk works nice and doesnt bite too hard.) You will find more wet rusty metal under each layer. If you are lucky you will get down to shiny pitted metal and it will not be too thin.

Use the acid on shiny metal as mentioned above and then rinse it and shoot with a high zinc primer. Coating with zinc is almost as good as galvanizing the metal. Maybe you can find a weld-thru zinc primer that is compatible with your paint: after all it is impossible to paint inside a spot welded joint.

I found that Eastwood rust encapsulator sticks to surface rust well but not shiny metal, pitted or not. They have a converter chemical pair too, the encapsulator by itself does nothing to the rust itself, it only seals it.

The 'rust converter' stuff in the spray can that turns surface rust black seems to work sometimes. It is holding up a year later in a place where I used it as primer and painted over.

I did this wrong a bunch of times before I started to get it right. Use your head and wear a respirator.