Cove base trim is vinyl baseboard, usually installed with vinyl or laminate flooring. It's available in a limited range of neutral shades, but can be painted successfully if you want a different colour. To make painted vinyl cove trim resistant to the onslaught of mops and vacuum cleaners, it needs to be cleaned, sanded and primed before it is painted to produce a durable paint job.
Clean the cove base trim with a heavy-duty cleaner and degreaser, such as trisodium phosphate (TSP) Unless you clean off all traces of floor wax, oil and grime, the primer and paint won't adhere well. Since you'll be putting painter's tape along the floor to protect it, clean the floor as well, so the tape sticks.
If you are painting new cove base trim before you install it, you won't need to clean it. However, in either case follow the cleaning with a light sanding with a sanding sponge or 120-grit sandpaper to rough it up a little and provide "tooth" for the primer to grip.
Before you start priming, very carefully tape the edges of the cove base, both along the floor and along the wall. Use blue painter's tape instead of regular masking tape because it does a much better job of preventing paint bleeding under the edge.
There are several different types of primer designed specifically to adhere to hard-to-paint surfaces, such as vinyl. Some are even water-based, for easy clean-up and low odour. If you plan to paint cove base before you install it, consider purchasing spray cans of both vinyl primer and paint from an auto supply store--designed for painting vinyl seats and interior trim, these products also work very well for cove base.
Most paint and hardware stores carry high-adhesion primers in both brush- or roll-on formulas and in spray cans. Spraying cove base that has already been installed will require an inordinate amount of careful masking to protect adjacent surfaces from overspray; brushing on the primer and paint will be your best option. Follow label directions for application and drying times.
As long as you have done a good job of preparing and priming, latex (water-based) paint will be plenty durable on cove base trim. It is also easier to use than oil-based paint, and easier to touch-up later. Use a high-quality, washable paint. You should only need a quart for even a very large room, so don't skimp on quality.
Apply it with a good-quality, angled sash brush. Since most cove base is 4 inches tall, a 2.5-inch sash brush will be perfect. Don't try to do the job in just one coat; two thin, even coats look better than one thick coat and will hold up better, too. Remember that water-based paint takes up to a month to cure to its final hardness, so be gentle with your cove base trim for about 30 days.