Anything you put on would wash off in the rain ... but you could try some really hot spices. Bit harsh but it'll do the trick. Or, wrap some aluminium foil over the wires, most of them don't like chewing foil. I've also used a medium tack double sided tape in the past - unpleasant for them, but stops them Or, you could hide out, wait for the dog to start chewing, and then correct the behaviour. That might take a while tho