There is an unorthodox way of 'cleaning' them. You could try sparking them. Have the relay switch repetitively, on a function generator for instance and have it switch an inductive load, like another relay, depending on the contact current capacity. Let it spark for a an hour at 1s interval.
This is more of a last resort, really. Buy new ones is my advice.
It looks like they are general purpose relays. They are often mechanically sealed but not hermetically. Real signal relays are hermetically sealed or even filled with an inert gas. You can tell by looking at the bottom. Hermetically sealed signal relays will look potted.
Happy sparking.