Most contemporary furniture builders use a mixture of veneer and hardwood solids. Walnut is no exception, and even though certain pieces of furniture appear to be solid throughout, there's probably more veneer than you would expect. Veneer is a thin layer of real wood, usually about 1/16 inch, bonded to plywood or composites. If the veneer has absorbed moisture, or has been exposed to sunlight or heat, the glue can lose its grip, or it might also be caused by a factory defect. Other problems are gouges and scratches. Deal with them using ordinary products and techniques.
Scratches and Gouges
Color the scratch using a walnut stain marker. If the scratch remains visible, move to the next step.
Sand the scratch using fine-grit steel wool. Sand lightly in an oval shape, parallel with the grain until the scratch disappears. Don't use too much force -- if you notice that you've created a small dip or depression, stop sanding. Brush the area clean with a soft cloth.
Apply a small amount of walnut oil-based stain to the sanded area with a soft cloth. Rub it in gently and then wipe it off. Allow the stain to dry. Spray a light coat of aerosol lacquer over the sanded area. When the lacquer is dry, rub the area forcefully with a piece of denim to polish the finish.
Apply walnut putty crayon to gouges. Hold it like a pencil and rub the tip of the crayon over the gouge, allowing it to fill the crack with the crayon residue until flush.
Rub over the area with a piece of denim to remove excess crayon residue from the gouge and the surrounding area. Finish by dampening a cloth with walnut stain. Use it to polish the area shiny.
Loose Veneer Repair
Insert the tip of a putty knife under loose veneer. If that's not possible, such as when dealing with a bubble, use a craft knife to cut a small slit in the bubble approximately 1 1/2 inches wide.
Pry up on the loose veneer with the putty knife and inject glue under it. If you've cut a slit, pry it open with the putty knife and inject glue into the slit by inserting the tip of the glue bottle into the slit. Spread the glue under the loose veneer with the putty knife. Insert the putty knife into the slit and spread the glue as much as possible.
Place plastic wrap over the glued area. Place a block of wood over the glued area on top of the plastic wrap. The block of wood should be at least 1 inch bigger than the glued area on all four sides.
Place clamps on the block. If that's not possible, place heavy objects on the block. Allow the glue to dry overnight and remove the clamps.
Scrape off the dried glue using a chisel. Color any whitish areas along the edges, or along the slit, using a walnut stain marker.