Painting over any finish without prepping and priming the surface is a bad idea. Previous coats applied to the chair may react with your new paint and cause bubbling, peeling, or your paint not to stick. Instead, you'll need to prepare the surface prior to any painting. Amateur Method: Take a medium grit sandpaper and have at 'er, this will give your paint a surface to stick to and remove some of the previous finish. Move your way up to a fine grit to remove any groves and smooth out imperfections. Wipe furniture with a damp cloth to remove debris. Paint a dry surface first with a primer, one coat should do This will increase paint bondage and increase the uniformity of the top colour. Paint your desired colour on top of the primer once dry. Apply as many coats as to achieve desired depth of colour finish. Professional Method: Your first step is to strip the paint from the furniture, this can be done with solvents (use caution as they are toxic, caustic, flammable, and kill kittens on contact), after the previous coating has been removed follow the same steps as the Amateur Method above. Good luck!