After a reasonable clean up (but don't grind all the rust off , just the majority of the crusty stuff as you need some surface rust for it to be effective and the metal will be also be weaker) I use Dinitrol RC900 followed by a coating of Dinitrol 3125, finished off with Dinitrol 4841.
If you Google them, you can decide if they are suitable for your use.
You can also use a nozzle to spray the RC900 through the cut outs in the chassis rail to arrest any corrosion around the subframe mounting points in the chassis rail where they usually rust. You can spray rust proofing wax coatings in their as well, but if any welding is required at a later date, then heat and some wax based products can set your car alight, so it's worth checking the spec sheet before you apply any into the box sections if subsequent welding is a possibility.