Old school stuff in the junkyards here look like that. Sun just bakes it right off. Usually the wind blows after it rains so the rust they do get is light even surface rust and not pitting. Paint shops here would sand that with 80 grit DA and then maybe clean it with PPG DX metal prep or similar before primer coat.

No dipping tanks here that I know of.

A few sandblast shops, but they do steel stairs and such for a living, and not old cars. You would need to make sure they can handle it first.