Alodining the rudder horn, trailing edge. main spar and skins. The Alumiprep seems to be doing its job - the aluminum came out nice and clean, but I am not happy with the Alodine It took far longer than it should have to obtain a reasonable "golden" color, and the last skin treated came out very patchy.
Just to be sure, I think I will apply a coat of primer to the rudder interior. That, and I will likely buy a new batch of Alodine for treating the horizontal stabilizer. .