What he said, if its a 2001, just covering up the rust with undercoating won't do any good. If it was not coated from new, then you have to fix the rust, then coat
Oil sprays are not too bad, to stop the spread, but you have to repeat it because it wears off.
I'm in the process of doing an underside rust restoration project on my 2004 in the below thread using the Eastwood products. Have only done the inside/outside rockers using their internal frame coating as can be seen in the photos. Also refurbished the running boards and have a few other outside/house projects slowing me down. I best hurry it up if I'm going to beat the cold/wet weather.
Repeat every 3-5 years, fill all cavities, doors, rockers, everywhere,,, did this with my 00 WJ, when i bought it rust free, had it redone after 3 years, Jeep was 98% rust free, 2% front axle lower spring perch when wrecked recently, and here in Norway they really salt the H out of the roads
2004 Grand Cherokee Overland 4.7 HO Quadradrive
95 ZJ Limited 5.2 Winter Project
Sold 94 ZJ Laredo 4.0 Winter Beater
R.I.P. 2000 Grand Cherokee Limited, 4.7 V8, Quadradrive, Shale Green Metallic, Agate/Carbon interior, Well modded and highly missed :(