I have used POR-15 as well as Rhino Liner as undercoating. I know the latter says it's not to be used as such, but it's worked very well so far to coat the rockers on my Jetta. I tried to strip as much of the factory rubber undercoating off as I could as moisture had gotten underneath and rust had started to form.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)