Thanks for all the nice comments! The advice/info from this bulletin board was very helpful.
I think the biggest mistake I made in this project was trying to clean the end caps manually. It took a huge amount of time and the end result was far from perfect. In the end, I didn't actually get the surface prepped for painting as well as I wanted and I suspect I'll have to re-do those pieces in the next couple years as a result.
I've never media blasted anything before, but i think that's how i'll clean the end caps next time around. There's a media-blasting company 10 miles down the road. It should cost ~$20 for the end caps and will save ~5 hours of labor. Even if the end result of media blasting were roughly equal to hand-cleaning, it's still worth it if I value my time at >$4/hr.