When you use a set period of time, like one hour, and your task is incomplete, use the continuous tense. I painted the wall for an hour. - means the task is finished. You are no longer going to continue painting.

2. is correct. It's difficult to explain why 1. is incorrect. Maybe it 'could' be correct, but only in very strange circumstances. i.e. if one was somehow describing past activities of painting the wall. Sometimes it's better to not worry too much about exact rules and instead just see examples to get familiar with the language.