Your Flex 3401 is going to be the machine to use on this project. You'll want to aim for clean and shiny only, putting any thoughts of correction out of your mind completely. If you don't heed this particular piece of advice and go after this paint with an abrasive compound, you'll likely learn a lesson that you won't soon forget.
I've done a lot of work on these panels, tested the limits of the paint, different machines and many different types of products. The absolute quickest performing product I've found to achieve clean and shiny on these trailers is 3D International Deep Blue Metal Polish.
I know it's not sold here at AGO and I rarely mention products not sold here but it is the magic on these pre-painted panels and rivets alike, but will need to be followed by an LSP of your choice. This stuff works so fast that it's honestly worth following it with a wipe on walk away sealant such as Blackfire Crystal Seal, Optimum Opti-Seal or Ultima's Paint Guard Plus. The WOWA application is the fastest way to go.
Duragloss #501 Marine and RV polish is a close second to the 3D metal polish on these and it has it's own durable sealant right in the product.
#501 has very good cleaning ability for working on these trailers but doesn't work as fast at the non abrasive metal polish mentioned above, plus you have the light abrasive in the #501 to contend with. The abrasive in #501 is light but it does take things from a super easy no brainer process of buff, wipe and move on to a process that requires some abrasive break down time and buffing time control from one work section to the next because it doesn't deep clean quite as fast, and the included sealant in the product tends to want to seal in the black that's coming off of the rivets. The work and wipe off while still wet process becomes more critical when working with #501 and add in the need for more process control and abrasive breakdown time, there's a steeper learning curve when working with this product but it does work well once you dial in your process.
You'll be surprised at how inefficient the use of traditional automotive compounds and polishes can be when working on these trailers. For one, they cut too much and second off, their chemical cleaning ability is all wrong for the type of crud you find embedded into the matrix of these pre-painted panels.
I think a lot of the crud that gets in and on these trailers has a good percentage of metal tarnish in it, both from it's own metal structure, and from the high percentage of metal rolling down the highway. There's like a water, grease, mud, metal, rubber slurry splashing around from one big rig to the next during wet highway driving conditions. The fact that these rigs don't have a protective clear coat protecting the paint like cars do, just invites all this slop to absorb into the paint on these rigs.
Thoughts? Questions?