> building a pushbutton on a pcb

> Maybe some sort of conductive coating for the copper?

Gold - if you can get it plated (try a jeweller, they may do it for
nothing if thrown in with a normal EP load)

Silver - perhaps track-repair conductive pen ? Ditto for plating

Electrical grease to keep air away from the copper. It might be
possible load the grease with aluminium powder to help

Pushbutton contacts that self-clean, like the curved finger ones
on slip rings

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