If you do not clean and sand the paneling, the paint will peel away rapidly and you’ll have to sand and start again.
Cleaning
Old walls gather dirt, dust and grease. With paneling, it's tough to see the buildup of these things until after you've painted and the walls begin to peel. In some areas of your home, moisture may be an issue as well, and mold and mildew may be growing where you can’t see it. This will show through the paint if not cleaned away. Paneling typically has a coating of polish or wax-based material to give it a sheen. Cleaning helps remove some of this coating before painting. Use a strong cleaner such as trisodium phosphate (TSP), or an ammonia-based cleaner. Rinse thoroughly with clean water when you’re finished.
Sanding
The coating that seals paneling and gives it a shine is not likely to come away with cleaning alone. Even if the walls are older and the sheen is gone, some of the finish may still remain. Paint can’t bond to this surface and peels away as soon as it is dry. For this reason, sand the paneling with fine grit sandpaper. The goal is to provide the primer a good surface to adhere to. Once you have removed the glossy coating, clean away dust with a damp cloth.
Priming
Before priming the entire wall, test a couple of different spots to ensure the primer will adhere and cover the wall adequately. Leave the primer to dry for at least 24 hours on the test spots, and then stick a piece of tape over the area. Rip it away and if you see no paint, the walls are suitably prepared for primer. If there is paint on the tape, you may want to clean and sand once more. Priming is an important step in painting paneling. First, it reduces the amount of paint used to cover the wall, and second, it prevents knots or stains from bleeding through your paint.
Grooves
A common error when painting paneling is filling the grooves before priming. Primer provides a good base for wood putty and ensures good adhesion. After priming, you can patch holes and gaps the primer has revealed as well. If you don't properly fill the grooves, cracking may occur, which can result in peeling paint. In areas where the sheets of paneling meet, use fiberglass mesh tape to prevent cracking after you’ve painted. Sand the patched areas smooth and apply a second coat of primer to the wall before painting. Grooves don’t require filling in order to paint. Often, painting over them and allowing them to show through limits the potential for cracking and peeling.