[NOT FOR THE ECO-FRIENDLY]
Hey guys I wana shared answer to all my frame and underside jeep rust.. Ok here we go.. Easy as 1,2,3...
1. Change your oil, yes you read it correctly..
2. Find a large old paintbrush (or even better, a paint gun) and lather the underside of your jeep with the used oil...
Hit everything liberally..
Hint: skip the muffler or you may get some strange looks the next time u drive down the road
3. Do it again the next time you change your oil.
This has completely stopped any and all the rust that was beginning because of road salt... It really works!!