A problem with wax seals on letters in films
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Maybe I'm a dope but I always thought it was because they were using rag paper in ye olden days. It's plush with loose fibers and lots of room for wax to get in there. Modern paper - especially high sheen etc - is too smooth and glossy. Wax just sort of sits on top.
In the video the guy literally applies the wax to modern paper and it works as he described. Unless I'm misunderstanding what you're referring to.
I think OP now sees the light, realizing because of the video that their prior assumption was incorrect.