A slow speed buffer with a lamb's wool bonnet will do the job. Many buffing compounds are available that will enable you to take the surface finish as far as a mirror finish. It all depends on what you want.

Eastwood at 1 800 345 1178 or Eastwood.com has all the materials you will need. Some auto parts stores that sell the automotive finishes and prep materials will also have what you need.

Tripoli compound will work well and quickly, giving a high gloss. For a mirror finish, follow up with white or red rouge polish. Both of these finishes will probably be smoother than the new aluminum running board, so you may want to hit it with the same compounds to match the polished older one.

Gard

If you are using a 7" buffer instead of a 9" you can probably get by with two pads. The wool pads are very durable and will last longer on diamond plate than the foam pads commonly used on paint. If you are going to do a whole trailer buy couple of extra and take back what you don't use. The wool pads can also be washed and reused.

The tripoli comes in a hard stick. You load the buffer by toggling the on off switch and jamb the compound stick into the outside face of the pad while it is slowly rotating. The compound is what does the polishing, not the pad. You will have to constantly add more compound, (every two or three minutes). Again, if you can return the extra, purchase at least one compound bar for each running board, extra for the rest of the trailer.

After your trailer is polished, an application of a marine wax/finish will keep it bright for quite a while.

Gard

Just a thought. If this is a job you really don't want to do you might check with your local truck stop. Quite often there are some guys that work in the service department of the truck stops that polish aluminum. Fairly reasonable priced as well.

I firmly believe that I have polished my last piece of aluminum, but you know what they say, never say never.

Try www.perfectpolish.com