The materials are simple:
Glass mat: bondo 8 sq. ft Fibreglass Mat | Princess Auto
Polyester Resin: bondo Fibreglass Resin (3.5L) | Princess Auto
Filler: Like bondo to fill and roughness that is left after grinding. I did not use any.
: Let me know what you find, I have yet to paint
either of the trailers that I did the belly band on.
Tools: Most important: disposable cover alls that have a hood, gloves, and goggles
Grinder: A small cheep grinder, or two. I use one with a flapper disk to gouge, and one with a flat sanding disk to smooth to a near flat surface.
Sander: I use a 1/3 sheet orbital sander.
A Dremel tool with a cut off disk. To cut the metal plates that you may, or may not choose to remove.
Fibreglass is easy. If you grind off too much, then put more down. If you put down too much, grind it off.