I have had good luck with trim that has lost plasticizer with the Mequiars natural shine protectant. Back to black works, but can take a few coats if UV damaged, and I usually reserve it for outside trim. The sanding will certainly work if nothing else does, but dawn is actually a little strong of a detergent for old vinyl as is the orange stuff.

Dollars to doughnuts you stripped the surface plasticizer with the Dawn.

I would recommend against Armor-All (IRMV); and vehicles I have used it on either makes them shiny/slick, exacerbates the chalky problem, or gets weird if water gets on it later. (Again, I have had good luck with Meguiars in this regard.)

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72% 1992 FJ80 + 17% 1983 OM617 + 10% 85/87/92 4Runner + *Eclipse/GMC = 100% Truck
(*<1%)
Build Thread: http://forum.ih8mud.com/threads/a-humble-fj80-om617-r151f-swap.759554/