@ Densetsu,
No the way i want to do it eventually whenever i get the time, maybe this summer, maybe in 2 years, is take out the axle, front subframe and all that stuff, then sand it all down, and apply truck bedliner or chassis saver which will last a long long time.
The rubberized undercoating i put on now lasts long, but its the winter salts and gravel s*** they put out here in Pennsylvania which wears it off.
I just respray it like every year like i said, and you can do it right over the rust, it wont aggravate it, it'll just stop it in its tracks cause no air/water etc can get to it and basically you stopped it. Mine was mostly surface rust but i want to keep the path for a very long time so which is why i did it.
Plus like 15-20$ every summer didnt kill anyone and basically its like 2-3hrs of your time, and once its done your underbody looks better than new.
Also dont know where you reside but since its the winter your underbody is dirty as s*** so i would wash it first or take it to a carwash and choose one of the options where it washes your underbody too.
Then go home let it dry off, and get at it.
Heres the thread that shows some pics i took when i did mine like 1.5 yrs ago,
post5110774.html?hilit=underbody#p5110774
Tell me if you need anything else answered.