Nice story about "sincere", but, not true, according to the dictionary people atMerriam-Webster Online:

" The "without wax" derivation of sincere is one that we've heard for ages now, it seems, but there is no evidence that sincere ever had anything to do with craftsmanship, statues, or wax. According to our sources, sincere is from the Middle French sincere, meaning "honest," which in turn derived from the Latin sincerus, meaning "whole, unsullied, pure, honest, genuine." Sincerus most likely derives from sem- or sim- (a Latin root word meaning "one") and –cerus, which is most likely akin to the Latin verb creare, meaning "to create." We can say it is clearly not related to the Latin cera, "wax." We have ample evidence of the Latin sincerus through Classical and Medieval Latin, and it has never been used to describe anything having to do with "wax." Sincerus has meant "honest, pure, genuine, whole" since the classical era. "

This is all by way of saying that the "without wax" story is just that—a story. It is a good one and makes for great copy, as they say, which is why it is so oft repeated. But unless we see some new evidence of early uses of sincere that make the connection between honesty and wax clear, we have to reject this explanation.