Welcome to Oldgas. Most like to clean a sign as well as possible without compromising it's integrity as far as paint or finish goes. Since porcelain is essentially glass it is pretty impervious to cleaners and the finish won't come off by simply washing it. A lot of old signs will have paint overspray or other things built up on them that won't just wash off with soap and water so you will have to use something that will remove those things like thinners or rubbing compound. Mild abrasive cleaners like Barkeepers Friend work pretty well to removed rust stains and heavier contaminants and even fine grade steel wool will work to remove old paint overspray. If it has been painted over or is covered with a lot of paint you can use paint remover without harming the porcelain....GB