"AHilton" wrote: (clip) Polyurethane don't mix. (clip)
^^^^^^^^^^^
I recall from the days when I was a paint dealer, that polyurethane had a
problem with adhesion to itself, because it dries so hard. So, if the
recoating was done more than a day later, sanding was especially important.
Urethanes also dislike sealers containing stearates (soaps). But, once a
lacquer film is dry, I do not recall any special reason why urethane would
not adhere (to a properly sanded surface.)

There is often confusion due to the fact that lacquers act like paint
remover over oil based paints and varnishes. In that sense, I can see that
lacquer and urethane do not mix. Even there, however, if very thin coats of
lacquer are misted on, allowing the film to build up while releasing the
solvent, it is possible to create a barrier which will allow the lacquer to
adhere and not damage the undercoat.

Does any of this jibe with your understanding? Have things changed since I
learned about it, or is my memory slipping.