eh, it's my understanding that the time to apply rust-proofing is before the rust starts. Suggest you will have to clean all the current rust before applying - and then the questions of how well the rust was removed will remain. Further suggest that applying rust-proofing over material already rusted will only accelerate the problem - perhaps mask it for a bit longer - but the rust will eventually show up bigger and badder than if left alone.
No personal experience - just speculating.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.