Well I got a really nice Honda Mitsuba horn that will re-plate beautifully but it was completely dead and non of my methods described in this thread worked to revive it

So put my thinking cap on and used the rig below which zaps a few hundred volts of high frequency AC (12v-> high volts inverter) across the horn terminals via a 33 Ohm limiting resistor. The inverter was modified as the output in normally 50Hz but I bypassed all that and simply used simply the high-frequency output from the inverter transformer Worked a treat and you get an audible fizzing when you get conduction and the corrosion is being eroded away. This is my go-to method in future for sure.
Had the inverter PCB module for ages but it basically came out of something like this one.
https://www.ebay.co.uk/itm/185519092553 [ Guests cannot view attachments ]