[paulsherwin]

The 'dirt' is usually oxidation.

You need to get the Servisol onto the actual contacts, which means finding some sort of gap or hole. Be sparing with it - there's no point in spraying gallons of the stuff about, especially if it's not getting inside. It does dry off over the next day or two, but you should wipe off any obvious surplus with a rag after applying it and exercising the switches.