In some instances, it is perfectly fine to skip the sanding step when re-painting a piece of furniture, but you will want to keep this information in mind when deciding if sanding would be appropriate or not.
Is the Surface Shiny or Slick? – If the surface is very smooth and shiny, you may want to use sandpaper as the paint or primer could peel off or easily scrap off with a fingernail. You will want to, at the very least, go over the furniture with 220 or 320 grit sandpaper; this will give the primer, or paint, something to adhere to.
Has the Surface Been Thoroughly Cleaned? – Make sure you start by cleaning the surface with some kind of degreaser before you paint or prime. Paint and primer will not adhere to any type of oil, even those used in cleaning solutions.
Has the Surface Been Waxed? – If you are painting over something that has a wax coating over top, sand the wax off. Paint and primer will NOT adhere to a waxed surface.
You Can Use a Liquid Sander – You can also wipe the piece down with a liquid sander (available at most hardware stores) instead of using actual sandpaper. NOTE: read the directions on the back before buying to ensure it will work with your piece, along with the primer, or paint, you intend to use.
Use an Oil Based Primer – If you decide not to sand, you will want to use an oil based primer, or a shellac based primer, as opposed to no primer or a water based primer. Oil and shellac primers cure faster, reducing the risk of paint peeling or being scratched off within the first 2-4 weeks.
If you read through this list, and still feel it is fine to paint without sanding, congrats! You’ve shaved some time off of this project.