The belly band on a Scamp
is aluminum. You don't need to polish it to get the reflective tape to stick, it just needs to be clean and wax free. I'd use a plastic tuffy and a soap that removes wax... like Tide. Just know, it will probably remove the wax from the trailer too.
You're removing the caulk that's currently on the trailer? I guess that's what you're meaning...
All caulk is messy, it's the nature of the beast. But you can make it less so by using blue painters tape close to the belly band and on top of the belly band with just enough space between the two to run a bead of caulk. Only run a bead that's adequate for the space. Depending on what you use, you can wet your finger to smooth it out... or use the back of a plastic spoon, etc. Remember, less is more when it comes to caulk in this area, it's for asthetic purposes, not for sealing possible leak points.
I like this stuff: Geocel Pro-Flex 10 oz tube sealer