To me every time I see a porcelain sign with a rusty back, a "red flag" is raised. The backside was the protected side, it should for the most part always be the better side. I have seen them with small chips that became rusty, but not the whole thing. Vintage and antique signs, that I have seen and read about (a lot of you deal in signs and see a lot more than I do) the process ensured that a "full measure" of porcelain was used on both the front and back. Some of the fakes I have seen in antique stores, have a porcelain front, but bare metal back. Saves money and gives a rusty back.
According to the "Encyclopedia of Porcelain Enamel Advertising" by Michael Bruner 2nd edition 1999, very old porcelain signs can and will have many small spots where there is no porcelain. "Some signs may have over a hundred of these small spots. This would indicate a sign of pre-1930 vintage, as manufactures used methods that eliminated most of these spots after that time. This technique is for signs with the advertising only one one side."
Chris, as I said before I'm still a rank novice when it comes to signs, so I do what you are doing asking questions here and I picked up a few books that deal with signs. I mentioned Mr. Bruner's book, I also use "A Collectors Guide to Porcelain Enamel Advertising Signs with Prices" by Bob Alexander 1990 and Richard Amistadi's book "Amistadi's Antique Advertising -- Gas, Oil and Automotive Signs Volume One" 2005.
Each one of us has to decide for them self if an item is real or fake. But making a decision with all help possible, makes a mistake a lot less painful, IMO.
Looking for Tide Water/ Tide Water-Associated/ Tidewater items