I found a very good method to fine tune so that the skins sit perfectly flat against the bottom skin. I used the same hand made tool I used to straighten rib flanges. The tool is a simple piece of wood...birch in this case. I cut a slot on both ends and radiused the ends so it would not make a sharp bend. On one end, I cut the slot at an angle so that I could have better leverage. I was very carefull on the center section and it took me several tries. But on the inboard end, I was a bit more agressive and the skins ended up tight after the first try. Now that I have the procedure down, I may consider tweaking the rudder skins.....or should I leave good enough alone!
