I'll probably undercoat mine since they put salt on the roads here
Upstate NY. checkin' in....:sobbing:
Mine came out of the south at 95K on her, it saw two?, three? winters, fortunately I don't drive much so those miles were easily under 10K all in. I then stopped all winter use and got an Outback for daily driver duties.
Still had a good scale going on, a bit of rocker panel rot on the drivers side front, since I park on a slightly downhill spot that also leans driver side, so everything ran and collected there, passenger side is fine. Had that cut out and patched in two years ago, front door only, rear section was solid (phew!), then did NH Oil on it that year too.
For $200, they shot into all the doors, rockers, frame, as well as sprayed the whole underside. Since it's tinted black, it looks SO much better.
It flaked off the big chunks, had a quick retreat last summer for $50 to re-cover/soak those areas, and I think I'm good for a while.
WaxOyl is another one, but way cheaper than say Ziebart.
It never really gets hard, which is I suppose a plus and a minus, but I'm super happy with the results.