Does semi-gloss need primer?

Semi-gloss paint does not always strictly require a separate primer if you are repainting over a previously painted surface in good condition that has already been primed and is the same or similar color, but using primer is highly recommended and often essential for a quality, long-lasting job. Primer serves several critical functions. If you are painting over bare drywall, bare wood, patched areas, stains, or a dark color with a lighter semi-gloss, primer is absolutely necessary to seal the porous surface, block stains from bleeding through, and ensure even color coverage. If you are painting over a high-gloss or semi-gloss surface, a bonding or adhesion primer is important because semi-gloss topcoats do not stick well to slick surfaces, and primer gives them something to grip. If you are switching from oil-based to latex paint, primer is also needed to prevent peeling. Even when not strictly required, primer improves adhesion, reduces the number of topcoats needed, provides a more uniform sheen, and extends the life of the paint job. For trim, cabinets, and other high-visibility, high-wear areas where semi-gloss is typically used, professionals almost always use primer.