I tried repairing my mould last night as there were a few pin holes in the surface.

Another method, that could also save you having to mix in the wax additive, is to create sticking plasters from flash or brown packing tape. By sticking a small square onto a larger piece of tape (so that the none sticky section covers the new gelcoat) you can seal-off the repair, which should allow full cure (as long as the catalyst is mixed appropriately).

If done carefully, you can also end up with repair sections that are just proud of the mould surface which should save time on flatting back also