According to the Boeing SRM (Structural Repair Manual), the scratches should be sanded down. But as the scratches caused by the tailstrike were too deep, the correct way to repair is to cut out the damaged section, then put a doubler plate that is 30% larger than the section that was cut out. However, in the repair of the accident plane, the workers did not follow the instructions. Instead they just placed a doubler plate over the damaged section, which was not 30% larger than the section that was supposed to be cut out. This weakened the structure behind the doubler. The workers then made one more mistake, by writing in the repair log that the repair was done correctly. So, anyone reading the log book would assume that the repair was done correctly and that the workers had cut out the damaged section and replaced it.