Do I need to undercoat over old paint?

Whether you need to undercoat over old paint depends on condition of old paint, color change, and surface type. For house interior or exterior painting, you do not always need to undercoat over old paint if old paint is in good condition, clean, matte, well adhered, and you are painting similar color, you can often paint directly with two coats of quality self-priming paint. You do need to undercoat over old paint if old paint is glossy, oil based and you are switching to latex, stained, dark color to light color, or has repairs and filler spots, as undercoat provides adhesion, blocks stains, and ensures even color and better coverage. For example, painting white over dark red old paint without undercoat needs many coats and still shows through, so undercoat needed. For car underbody, you do not need to undercoat over old factory undercoating if it is intact and well adhered, you can clean and apply new coating over it, but if old undercoating is peeling, cracked, or has rust underneath, you must remove loose material, treat rust, and then undercoat, otherwise new coating traps moisture. So for house, undercoat over old paint is needed for glossy, stained, or drastic color change, but not always for good condition repaint.