No. Primers are not always needed when repainting.
If you are repainting a similar color, you can lightly scuff sand to remove the gloss and clean the surface. Once that is completed, you can simply repaint over the existing latex coating. More than one coat may be needed for uniformity.
Please note that switching sheens or drastic color changes may require a primer to minimize the amount of topcoats used.
WARNING! If you scrape sand, or remove old paint, you may release lead dust or fumes. LEAD IS TOXIC. EXPOSURE TO LEAD DUST OR FUMES CAN CAUSE SERIOUS ILLNESS, SUCH AS BRAIN DAMAGE, ESPECIALLY IN CHILDREN. PREGNANT WOMEN SHOULD AVOID EXPOSURE. Wear a properly fitted NIOSH=approved respirator and prevent skin contact to control lead exposure. Clean up carefully with a HEPA vaccum and a wet mop.
Before you start, find out how to protect yourself and your family by contacting the USEPA National Lead Information Hotline at 1-800-424-LEAD or log on to www.epa.gov/lead. Follow these instructions to control exposure to other hazardous substances that may be released during surface preparation.