Should armor really be trainable?
Training shield handling makes sense as its an active skill, but armor is more passive in nature unless there is some sort of martial art that turns the use of armor into an actual skill.
In real life, If I put on full plate armor and let a crowd of people whack at me with sticks, I'm not going to be any more proficient with armor after a days worth of pummelling. In the first 10 or 20 minutes, I might learn to block the joints and uncovered areas, but this is fairly easy. It takes nowhere as much skill as it would take to master fighting.