D’ya know what I hate? I hate it when folks insist on spreading misinformation. I especially hate it when, upon being challenged, they support their position with, “I’ve been teaching this subject for umpty years, so I know more than you do.” I’m sorry, but I don’t think it matters how long you’ve been teaching something. What matters is whether or not you ever bothered to become sufficiently educated on the topic in the first place.
Hrmf.
Folks, do not use acrylic medium as a final sealing coat. It’s not a varnish, and shouldn’t be used as one. It’s not impervious, it’s not protective, and it’s not reversible. And, while I’m on the general subject–for the love of God, do not ever put acrylic paint or medium over oil pastels.