At that point it's probably just easier to replace the sockets. More expensive, yeah...but at least you don't have sockets where water-based paint has leaked inside the sockets and rusted the contacts out.
That's what happened with the old two-pin sockets in my flat. They had so many coats of paint (probably 50 years worth) that in some cases you couldn't shove a plug in. If you could shove iti in, it would not make contact in some cases. And when I went to remove them, I noticed the housings were cracked and came out in pieces!!!
Fortunately the system is grounded (through the metal sheathing on the cables) and I was able to replace them all with three-pin sockets. Otherwise I would have had to replace them with new 2-pin sockets....yup...they're still made for replacement use.
