You are trying for a "mechanical" bond. If you shoot multiple layers of gel coat, you want to do it before the first layer flashes off so that a chemical bond is achieved for the those layers.
And since the gelcoat that is already there is chemically cured, there's nothing left for the new gelcoat to bond with on the molecular level. So, you give it tooth...rough sand it. This gives the new gelcoat something to mechanically bind to.
