The zinc phosphate you describe that dries to a white powder when applied on rust sounds like the metal prep that POR-15 sells. The sell a degreaser and then a prep, then you put the actual rust blocker on. The stuff ain't cheap but seems to be very tough. I don't know if the zinc by itself would just wash off over time without some exterior sealant like paint or the POR.

It is what I use.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6