Ceiling paint can crack, peel or appear chalky if you don't prepare the surface properly. Not every ceiling, however, needs to be primed before you can apply paint. You can tell if your ceiling requires primer by looking at its existing finish, condition or material.
If It's New Drywall
A bare drywall ceiling is very porous, and it has uneven texture where joint compound meets wallboard. This combination equates to irregular sheen, or flashing, if you don't prime before you paint it. For the primer to adhere properly, let the joints cure first; cured compound is free of moisture that would otherwise bubble through the finish -- even if the compound feels dry, give it about a week to cure fully.
On smooth or professionally finished drywall, use latex drywall primer and then ceiling paint, or an all-in-one ceiling paint and primer product. If the joint compound appears pitted or poorly sanded, however, start with a high-build drywall primer-sealer to help level the surface before painting.
If It's Textured
Protect a drywall ceiling with primer or primer and paint before adding any ceiling textures. Texture, such as popcorn or the flatter knockdown version, typically doesn't need a primer finish, and only needs paint if you want to change the color. The bumpier the texture, the longer the primer roller nap needs to be to fill the hills and valleys effectively. You can use a paint sprayer instead, for better coverage.
If It's Wood
If you're going to paint a wood ceiling, plan to prime it first. Many types of wood naturally bleed tannins or ooze pitch, therefore requiring a shellac-based primer to control bleed-through discoloration. Shellac primer should also be your choice if you plan to paint a tin or plaster ceiling. Latex primer is fine for pine or other no- or low-tannin, bare softwoods.
For varnished, stained or weathered wood, use oil-based primer, but not before sanding off at least a little sheen and any loose bits -- protect yourself with safety glasses and a dust mask.
If It's Painted
If the existing paint is water based, and ceiling paint typically is, skip the primer and simply prepare the surface for new paint by cleaning it and removing flaking paint. If it's oil-based, however, you cannot apply water-based paint over it without first applying a bonding primer. The bonding primer may be latex or oil based, but as the name implies, it bonds or grips "slick" but clean, tight surfaces.
If you aren't sure if the existing ceiling paint is oil-based, wipe a little non-acetone nail-polish remover over it, using a clean rag. If paint color transfers to the rag, it's water based. To reduce the coats you need when you change the ceiling paint color, have the primer tinted to match.
In homes that built before 1978, have your local health department test the paint for lead content before sanding or dealing with flaking paint.
If It's Stained
For stains, such as kitchen grease splatter on a painted ceiling, thoroughly wash the ceiling first. If stains or a greasy feel remain, spot cover them with a couple of coats of stain-blocking, shellac- or oil-based primer. Allow the primer sufficient drying time -- typically 15 to 20 minutes, depending on the room's heat and humidity -- before repainting the ceiling.
Water stains, from roof leaks or from a bathroom above, for example, require stain-blocking primer -- after you've fixed the leak and removed any mildew or peeled paint. By using the right primer for any ceiling, you'll end up with an even finish, no bleed-through and no need for a redo.