Recently I painted over the wood paneling in the downstairs apartment of my new house. Sadly, no prospective tenants cared for "Brady Bunch chic" in my ad on Craigslist. I couldn't afford to rip it out and start over with drywall, so I had to figure out how to paint it.
While wall prep is always the most important part of a paint job, that's doubly true for wood paneling. That's because the surface has not only been treated with some kind of varnish or oil to seal the wood, but it probably hasn't been washed down since whenever it was originally installed. Who is going to up and wash the wood walls at spring cleaning?
Here are the results of my "lessons learned." Hope this saves you some time.
Take a hammer and a handful of brads around the room. Make sure the existing brads are flush with the surface of the wall, and tack down any bowed, buckled or loose panels. The picture shows a lose panel. The misaligned edge of the board is very noticeable.
Check out the seams in the wood paneling. You may want to fill them with a paintable adhesive caulk, especially at corners.
Remove any random screws or nails you discover and use wood putty to fill the holes. Don't skip over this step: holes that are practically unnoticeable on wood paneling become quite visible when paint evens out the surface reflectivity. Sand down the excess wood putty after it dries.
Wash the walls down with Tri-Sodium Phosphate, or TSP. The powder is a better value and you can mix it in a bucket of hot water. Make sure you wear rubber gloves. Go over every inch to make sure you get the spider webs, the sanding dust, and accumulated grime from the surface. I recommend washing in line with the wood grain. Don't be shy about dumping the bucket when the water turns dark - the goal is to take the dirt off the walls, not spread it around.
Invest in a Sander/Deglosser solution. It's more expensive than TSP but significantly less costly than primer, and it's easier on the elbow grease than taking sandpaper to every inch of the walls. Rub the deglosser liberally into the walls with a clean rag, folding often (as you will probably get some yellowish surface grime on the rag). Again, make sure you wear gloves. Seriously - don't skip this step unless you want to spend money and time on a second coat of primer. The picture below shows how my panels looked like before I started.
Tape off the ceilings, floors, windows and anything that's not getting painted. You know the routine. The better your edges, the less time you will spend scraping paint with a razor blade.
Prime. A friend of mine insisted that only an oil-based primer should go over wood paneling, but I took a chance on latex - which is less expensive and significantly less toxic - and it was great. There are different schools of thought on whether you should dive in to rollering the walls or start by cutting in with the brush, and I'll leave that up to you. However, I strongly recommend that you take a foam brush and prime all the grooves before you start with the roller. The roller is never going to get "in" there, and then those bare grooves are going to taunt you and lure you into going over the same patch five times, which is an inefficient use of time and primer. As you paint stripes over the grooves, be sure you feather the edges so you don't wind up with drips and variation in the thickness of the primer coat.
Finally, roll primer evenly over the walls. If you did a good job with the deglosser, one coat should suffice. It doesn't have to look like a pristinely white surface. You're aiming for something that looks like a sheet or two of white tissue paper over your wood paneling. If you have bald patches where the primer just isn't sticking as evenly, you need a second coat.
Once you've got an even primer coat (don't worry about stripes where you primed the grooves), your wood paneling is like any other wall to paint.
The paint has extended the life of my paneling by a few years. A solid, defect free surface is much more pleasing to the eye than old, faded, fake wood texture.