old auto parts lighted clock sign

I found this sign in a old auto garage just outside fort lewis wash.with a few other items.I don't have much knowledge about this sign but I like old cars 60's 70"real hot rods. so anyway i'm a rookie this clock is full of dirt from being hung on a wall for 40 years I was wondering if I should clean it up or will that hurt the value or what something like this might be worth if anybody can give me some advice it would be helpful