Finishing over paint

For a completely clear finish, I'd use a water-based poly.

Oil based poly or varnishes will give a yellow discoloration to anything under it, and so will shellac. Also, the solvents in oil based or shellac might dissolve whatever is painted under ti (would depend on what was used).

I smearing might be a problem, then spraying on the poly (very light coats) would be preferable.
 
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