Your hull looks like it is original colors? Is there any paint on it, or is the metal flake mixed with the gel? If any of it is flaking off, you definitely need to sand it off so you have a good bond. If its solid, Id rough it up with 80 grit and go to town.
Im doing my boat right now. Im spraying gel. Going very well. Lots of sanding, but I think it will be well worth it. Paint will chip easily.