Whatever product you use there is no point in spraying it on the rust, you have to deal with that first.
Firstly it's vital you clean the inside of the chassis, then some sort of rust converter followed by a generous coat of cavity wax. Then you can deal with the outside.
Or you can just spray some black stuff on the outside after rust treatment.
You could ignore it all and then spray some stuff on it when you sell to try and fool the buyer.
Another option is weld it up while saving for a new chassis.
In the past 9 years I have cleaned, painted, sprayed the chassis many times, I am now welding it up to get a few more years out of it while new chassis fund is building up.