Re: gel coat worn through to metal flake
It depends on exactly how much clear is gone from the surface. Dull with no shine...sand and buff, if the surface is rough and you can feel the flake it gets more complicated. If you sand into the flake for any reason, or the flake is exposed, then sanding into it may make any repair look blotchy or different from the surrounding area when clear is applied over it.
Clear coats over metal flake tend to fail and start to peel over time, frequently its because of poor prep work. Although good paints tend to bond very well to gel coat, you need to be very careful to not sand into the flake, so if the clear gel coat is thin you don't want to remove it and expose any flake, this leads to the failure when not enough scuffing of the surface is done. Gel coat typically needs better surface prep, or I should say just more aggressive prep work with a coarser paper, which will remove more of the surface, which can more easily expose the flake.
The look of the finished product will also be affected by the color, size, concentration and thickness of the flake layer, sometimes sanding into the flake isn't as noticeable depending on these variables. You can also add some pearl to the clear you apply, this will hide some of the defects and bring back some of the color of the faded flake and/or gel coat.