Just to add a little to Warren's advice

When doing repairs to moulds where a bit of the Unimould gelcoat has broken away after a period of time, usually around any areas where the filleting wax was used to fill the parting line between patter and flange, I've found that filling with additional gelcoat doesn't always make for a lasting repair. Despite careful preparation, the mechanical bond will never quite be the same as the chemical one seen during the initial construction of the mould, I guess.

However, I have found that using the Epoxy filler that EC sells seems to offer a more robust and lasting repair, so I tend to use this now. I know that it might not be the recommended product, but you might find that this product works better when it comes to filling any join on the surface of your extended mould and possibly does a better job of adhering itself to a cured, keyed surface. It can be polished too, which should reduce the risk of anything sticking too much.and it's often easier to work into a repair, I've found 

Just a thought...