Do you need to prep a smooth paneled wall before painting?

Yes,lightly sand,clean ,apply two coats of primer allowing to dry in between and then paint.

Just make sure it's clean,, there so many paints out there with the built in primer, these paints are meant to adhere to pretty much any surface.

Another way to prep a paneled wall is to clean it and cover it with a thick, textured wallpaper. This helps change the design and hide the vertical stripes.

No sanding necessary. Wash it down with Dawn. Definitely use a coat of regular primer FIRST and then paint because the paint with primer does not cover it and you'll need to put more coats on then necessary if you don't use regular primer first. Use a tiny paint brush to paint the the vertical lines if you have them. Did this with my basement paneling and it turned out beautifully.

I did lightly sand my paneling, then washed to get any sanding dust off. Coated with Kilz, then painted.

I used my palm sander to lightly sand the paneling so the paint will adhere to it, then wiped it down and vacuumed. I did not use any kind of sealer, I just painted it and it covered in just one coat. Have fun!

I agree with Shirley.