I didn't use anything on mine.
I found that if you put the clecos in alternating sides, the flap will stay straight.
Then I counted the rivets and marked the center and end ones as #1. Then I divided those spaces in two and marked the center hole #2. I kept doing that until every hole was marked with a number.
Next, I ground down and polished a rivet set to match the angle of the flaps.
(click to enlarge)
With my modified rivet set, I carefully half squeezed the rivets. Inserting the three #1 rivets first, then the two #2 rivets, then the four #4 rivets, etc.
I also marked the rivets so that they would be inserted from different directions in each hole. Thus you would see a shop head, factory head, shop head, factory head, etc. as you looked down the surface. (Is that clear?)
After I half set each rivet, I went back in the same order and finished setting them. All my control surfaces are straight as an arrow.