Yes, some type of first layer of primer should be used, especially metal and resins. Plastic CAN soak up the paint (like acrylics) and lose the overall color.

Be careful chosing the primer though. Some really cheap ones must have several coats of paint or need to be sealed.

Plus, clean or wash the item before the primer/paint process to remove any mold release agents and even the oils from your hands.

HTH