"Carefuly" rub the area with a lint free cotton rag and Denatured Alcohol, the Denatured Alcohol will dissolve the shellac and the rag wrapped around you finger will spread the shellac around and into the defect. Think of spit polishing your shoes in the military and rubbing with the tip of your finger. You can also just rub on new shellac over the defect and it will blend in to the old area. If varnish was used then simply rub some wiping varnish on the area and this depends on what country made the SKS.
French Varnish below using alcohol and Shellac, you can buff the sheen off with 4 0000 steel wool. I mentioned actual varnish rather than shellac because I had some Communist weapons where shellac was not used and real varnish was. You have two methods depending on the type of repair, meaning rub out the defect with just Denatured Alcohol or apply new shellac and blend in.
The Secret life of Shellac -0r- Bugs are our Friends!
http://parallaxscurioandrelicfirearmsforums.yuku.com/topic/16581/The-Secret-life-of-Shellac-0r-Bugs-are-our-Friends#.VAsz5_ldV8E
Topic: Original Stock Finish on Russian SKS
http://www.sksboards.com/smf/index.php?topic=99474.0
http://www.thefreedictionary.com/French+varnish
http://en.wikipedia.org/wiki/French_polish