I did my whole C10 frame as well as smaller parts. I just kept it wet with it until all the rust had turned black. Depends on how much rust and how deep. Speed it up by removing the real loose rust before wetting. Soaking is better, and with control arms you can do half at a time. Be sure to remember rubber gloves. This stuff finds tiny cuts on your hands you never knew you had. But, it will let you know about each one in short order.
Good luck, and where eye protection of course.