One last thing you *might* try before giving up is a product that is sold here in EU under the name Chanteclair (I suppose you might get it via amazon or alikes, if there is no local equivalent). Despite its funny Marseille soap perfume that recalls washerwomen of bygone days, this is extremely powerful. It is - just to mention an application that I use frequently - much more efficient in removing old finger dirt deposited on radio knobs than gasoline itself.

Then yes, wait for some A.I. laser dirt remover or alike!
