The belly band itself serves no function after construction. It is used to line up the top and bottom half of the shell. Metal plates on the inside of the shell and the belly band on the outside. They riveted the plates and the band together. This pulled the top and bottom so that they lined up. Then they ran a fibreglass bandage over the inside. This covered the metal plates and sealed up the seam. From the outside it looks like the metal plates are in upper and lower pockets. The problem is that the lower pockets that the metal plates sit in, fill up with water, (condensation?). In colder climates, this water freezes and slowly opens this pocket up, till it ruptures into the inside of the trailer. Now there is a leak in the belly band.
If all you are doing is putting the belly band back on, then no prep is required. The metal plates rust and fail to hold the band on anymore. They will probably put new metal plates, (washers?) in. so that a new rivet can be used to hold the belly band on. I have not tried this, but I suspect it is not easy.
If your trailer has been exposed to a constant freeze / thaw cycle, all it's life, then leaks
may be a problem. There are several possible fixes. On the more extreme end is what I do:
My First Belly Band Thread
My Second Belly Band Thread
Others have just filled the gap with bondo, and reinstalled the belly band. I personally like the way that Trilliums look without the band.