Wall & ceiling prep before applying texture (orange peel)

What preparation is necessary on the walls & ceilings before applying an orange peel texture?
Walls: they currently have a painted knock-down texture. My contractor wants to spray an orange peel texture directly on it, but I'm concerned that it may not hide the old texture well. Will it work? Should I have him apply a skim coat of compound on the walls first? Is a primer needed before texturing?
Ceiling: after removing the old popcorn texture from the ceiling, it's left with bare wallboards (unprimed, unpainted). Again, my contractor wants to spray the o.p. texture directly on it. Is a skim coat required before texturing? How about primer?
Bullnose: my contractor bullnosed the wall corners, and his job left a lot of metal exposed. He wants to spray the o.p. texture on it directly. Will it stick? If not, what goes on the metal before spraying texture?
Thanks!
-Ernesto
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I would clean them with tsp before spray

Should be finish or refinish with new tape to get a good flat before apply texture I preffer to apply primer afterward

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