Sand the entire surface of the rough veneer wall using a sanding buffer with a medium-grit sanding disc. This will remove any large areas of texture and roughness.
Sand the veneer again using the sanding buffer with a fine-grit sanding disc to make the surface as smooth as you can.
Clear the area of any dust and debris caused by the sanding using a shop vacuum with a brush attachment.
Stir a container of joint compound to make sure the consistency is uniform, then scoop the compound onto a flat trowel.
Scrape the compound over any areas that are too low to create a flush, even surface. Allow the compound to dry for eight to 10 hours.
Sand the edges of the joint compound using fine-grit sandpaper to ensure that the compound is totally flush with the wall. Move the sandpaper in small, circular motions.
Vacuum the area again before applying any paint or wallpaper.