Primed all stiffeners and stiffeners rivet lines on the skin. Riveted one side of stiffeners to the skin IAW Van's Construction Manual. Frustrating day due to repeated error. About halfway through riveting the stiffeners, I missed the back rivet plate and put a nasty crease in the skin, after much review of the error, I was able to flatten the crease quite a bit using the flush rivet set and the back rivet plate. Continued back riveting the stiffeners and missed the !@?%$ rivet plate again! This time the crease wasn't too bad. Both creases should not be of structural issue, and can be filled with bondo to hide after painting. If they were to develope cracks in the future, the answer would be to rebuild a new rudder! Stopping now for today as I am pretty bummed about the mistake! Next time I will quite while I'm ahead!