My Father worked for RCA in Bloomington, Indiana most of his life and his job was to patch TV cabinets. His tool was a knife similar to a putty knife and it was heated with a small "oven" that was about 8" long, 3" wide, 4" tall and oval in shape. It had a slit horizontally through the middle where you placed the knife in and was heated electrically. This "wax" was, I believe a form of lacquer and when melted could be formed to fill scratches, gouges or even rebuild broken corners or edges. He also had several powders he used to match any color wood or plastic that was used on cabinets. This was in the 60's and 70's. He used his skills to repair damaged instruments like violins, mandolins, ect. and made many players happy over the years. 

Right on Rustynbent.  These sorts of tools are still available, and the filler is shellac.  Constantine's sells the kit:

http://www.constantines.com/burn-inkit.aspx