Zinsser Seal Coat question

I finally picked up some dewaxed shellac. In reading the fine print I noticed that there is a caveat about using it with moisture cured urethanes. BUT, I can't tell if they mean it is a problem putting shellac OVER the MCU, or UNDER it. What's the story?
dave
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wrote:

Test _your_ combination on a decent sized scrap board.
Barry
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