Stringbuilder not working

I'm trying to use a stringbuilder to add lines together if the line is shorter than 305 characters. It reads the first line fine, reads the nextLine fine and captures those.

For some reason when I get to the string myString part it's blank..  How do I get my results into myString? Thanks!

                            policyLine = line;
                            nextLine = sr.ReadLine();
                            if(nextLine.Length < 305)
                            {
                                StringBuilder sb2 = new StringBuilder();
                                sb2.Append(policyLine);
                                sb2.Append(@"\");
                                sb2.Append(nextLine);
                                string myString = sb2.ToString();                          
                            }