When repainting any surface, and the old paint is glossy it may need to be lightly sanded to ensure a good bond. On smooth surfaces to be repainted 220 grit sandpaper usually works well just to take down the gloss. The Oil /Alkyd paints, often tend to bond fairly well to a previously painted glossy surface without sanding. Painting latex over a glossy oil finish without some preparation would almost certainly lead to problems. Painting with quality latex over a semi-gloss latex will usually bond quite well without sanding.
De-glossers: A sometimes-easier method of dealing with gloss is to use a de-glosser. A liquid product available at most hardware stores available in quart cans. The de-glosser is not used until you are about ready to start painting; this is because it looses its affect after a shot period of time. To use the de-glosser; apply the liquid to a clean white rag, and wipe over the surface to be painted .As you wipe you should soon begin to feel the rag drag so that it dose not slide to easily, this is because it is softening the old paint. Do not over-wipe or it may affect the uniformity of the next coats gloss leaving a splotchy affect, and thus requiring a second coat of finish paint. Only de-gloss about enough area so that you will be able to finish painting the de-glossed are in a half-hour or so, and then you can start wiping the next section.
Sometimes de-glossers don’t do the job. If the rag slides easily and you do not soon feel that the paint is softening; the de-glosser may not be having enough effect to do much good. Most de-glossers on the market today are quite toxic and give off strong fumes, but there are some environmentally friendly de-glossers that do work very well in many cases.
Bonding Agents Some primers are designed to stick directly to glossy surfaces without dulling. They are available in both latex and oil base and can usually be used as a sealer also. They can be used under oil or latex paint
New paint When your working with glossy paint and you will be applying a second coat, you should not have to dull between coats. This is because the first coat has not hardened completely even though the paint is completely dry and hard enough for a second coat.