If you're wondering whether you can paint that old wood paneling to brighten up the room, the answer is: yes, you can do that and more. It doesn't take much effort to fill the grooves and make the surface so smooth and flat that no one will suspect it's paneling. Of course, if you happen to like grooves, you don't have to fill them, but whether you fill the grooves or leave them uncovered, the secret to a long-lasting job is to properly prepare the paneling. The last thing you need in your den or living room is peeling paint.
Cover the floors with plastic sheeting and move all the furniture to the other side of the room or into another room. Things could get messy.
Mask the walls and ceiling around the paneling with painter's tape and masking paper. The more thoroughly you mask, the less careful you'll have to be when painting.
Inspect the paneling for protruding nails and tap in any you find, using a hammer and nail set. Fill the nail holes with wood filler. If you plan to fill in grooves on the paneling, you can wait and fill the nail holes at that time.
Wash the paneling with a solution of trisodium phosphate and warm water. TSP is a strong detergent, so wear rubber gloves and goggles when using it. Mix about 1/2 cup of TSP per gallon of water, and use a sponge to wash with it. It will de-gloss the paneling and remove any grease and dirt that interferes with paint adhesion.
Fill grooves, if desired, with drywall joint compound -- also known as mud -- after the paneling dries. Trowel the mud into the grooves with a drywall knife and scrape it flat with the knife, removing as much excess as you can. Let the mud dry overnight, then apply more if the grooves aren't flat. Depending on the size of the grooves, it could take three coats to fill them.
Scuff sand the paneling using a pole sander and 120-grit sandpaper. This treatment etches the old finish even more, thus providing better paint adhesion. If you mudded the walls, sanding flattens and smooths the mud.
Apply wood primer. In most cases, you can use a water-based product to keep VOCs to a minimum and make cleanup easier, but if the paneling is veneered, it's better to use a shellac-based primer to avoid bubbles in the veneer. Apply the primer around the edges of the paneling with a paintbrush and use a medium-nap roller to fill in the middle.
Apply the first coat of latex paint around the edges of the paneling with a synthetic-bristle paintbrush. If the paneling has grooves that you didn't fill, brush along each groove to ensure it gets completely painted.
Roll the middle of the paneling with a medium-nap roller. Work vertically from the top of the wall to the bottom. Start at one corner and paint a column about 3- or 4-feet wide; then move over and paint another column in the same way. Continue until all the paneling is painted.
Allow the paint to dry overnight; then apply a second coat. Repeat one more time, if necessary.