If you're painting solid wood or hollow core doors you shouldn't need a primer. Just make sure to sand the surface to give the paint something to hold on to and that should be enough.
However, if you're painting a laminate door you will definitely need a good primer. You will probably still need to lightly sand as well to ensure an even smoother finish.
Another instance where you might need a primer is if you are covering over oil-based paint. You can check this by using a cloth dampened with rubbing alcohol. If the paint rubs onto the cloth, then you shouldn't need a primer. If it stays put, then you'll need to prime the surface before applying a coat of water or latex-based paint - even if it's the same color.
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