To clean relay contacts safely and thoroughly I've used fine sandpaper, but is there a spray, brake clean or something, that is safe and effective?

Relay contacts should not need any work. They may be oxidized from the effects of a corrosive environment, in which case you can use DeOxIt from Caig to clean them up. Switches are a different story: I have found that cleaning them up seldom works for long; that I am better off just replacing the switch.

In my experience I have never seen relay contacts get dirty or pitted or worn in any way on both my Volvos. This includes fuel, system, OD and radio supression relays. I have however had to resolder both OD and fuel pump relays due to cracked solder at the current carrying solder joints. The contacts are shaped to provide could contact, so I would not sand or file them. If they look good leave them alone. If you must pass a thin cloth or cardboard saturated with alcohol through the gap .
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David Hunter

As an electrician, we use something similar to a fine emory board (fingernail file board) to dress contacts until we are able to get a replacement set of contacts. NEVER use any material that has abrasive grains that can come loose and lodge in the contact space. This WILL cause contacts to burn and likely damage the device being controlled by those contacts as amp draw will be significantly higher to jump the gap caused by that particle.being where it should not have been.
Contact cleaner is available in aeresol cans at your local auto parts house.
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There are no stupid questions only those that are not asked.