The laws says the sign has to be "conspicuous" and to violate the law you have to "knowingly" ignore the sign. I submit that a code of conduct sign is not a conspicuous posting and if you hadn't read it, you would not "knowingly" be violating the law.
I can go with that i totally overlooked the "knowingly" part. When i make entry to a building im not looking for a code of conduct with 70 items and guns listed at #58, im looking for a sign specific to hand guns. (Not saying im going to ignore it if i see it, im just not going to look as hard at places other than the entrance.) I also revisited the definition of "conspicuous" in the ORC which reads as follows:
(10) “Conspicuous”, with reference to a term, means so written, displayed, or presented that a reasonable person against which it is to operate ought to have noticed it. Whether a term is “conspicuous” or not is for decision by the court. Conspicuous terms include the following:
(a) A heading in capitals equal to or greater in size than the surrounding text, or in contrasting type, font, or color to the surrounding text of the same or lesser size; and
(b) Language in the body of a record or display in larger type than the surrounding text, or in contrasting type, font, or color to the surrounding text of the same size, or set off from surrounding text of the same size by symbols or other marks that call attention to the language.
Paying attention to part (a) and (b) i think that rules out anything other than a sign that is dedicated specifically for no weapons because a code of conduct would typically not display the no weapons in a larger text or include anything to contrast it.