Can I just paint over semi-gloss paint?

You technically can paint over semi-gloss paint, but if you just paint directly over it without any preparation, your new paint will likely not adhere well and may peel, chip, or look uneven. Semi-gloss paint dries to a hard, slick, non-porous surface with high resin content, which makes it durable and easy to clean, but that same slickness prevents new paint from gripping. To get a good, long-lasting result, you need to do some prep work. The standard process is to first clean the surface thoroughly to remove grease, dirt, and grime with soap and water or a degreaser. Next, you should lightly scuff sand the semi-gloss surface with 120 to 220 grit sandpaper to dull the shine and create microscopic texture for adhesion. After sanding, wipe away all dust with a tack cloth or damp cloth. For best results, especially if you are switching to a lower sheen like matte or eggshell, apply a high-quality bonding primer or adhesion primer before your topcoat. The primer ensures the new paint sticks properly and covers evenly. Skipping these steps often leads to failure.