Ospho and Navel jelly have a few advantages over paint on converters,they actually REMOVE rust and convert any small traces left behind to iron oxide.
A few advantages Ospho has over navel jelly is its much stronger ,faster and does not need to be neautralized with water...I've personally never had a paint problem with it in 35 yrs and hundreds of cars.But then again I've seen people screw up using W&G remover so its not fool proof...
My personal opinion is ,if there's rust I wont prime if its not scrubbed down with Ospho first (its the law) even if a car is sandblasted it gets Ospho right away to protect the bare metal from surface rust
You can put it in a pump bottle and spray it on it can also be used as a converter and simply spray it on and paint when its dry...I dont use it this way but The guys that paint rusted metal roofing swear by it.
so if your looking for an ez spray on half measure fix you might as well save a bunch of money too Ospho only costs 15,00 a qt but for the bottom of a floor pan you should use a rubberized undercoating on top.