Just another tip that I always do just to make sure before I make a part.
I check to see how active the surface is with a piece of masking tape. Put the masking tape down onto the surface of your mold and then peel it off taking note of how easy it comes off compared to an untreated surface. You will soon know how easy your release will be. If it is too active you need to apply another coating of release or wax or whatever you are using.
Fred

If these are haze cracks and you are desperate to get just a few more parts out of it, you could just go with a few good coats of wax and then a number of coats of PVA.
Hope this helps.
Fred

Thanks for the replys.  I still had the masters so without the means to safely use 2 pack fillers etc it was quicker and cheaper to make new moulds.

With any semi permanent, make sure you put the cap back on it IMMEDIATELY. I have been using a semi perm for over 18 months now from the same 5lt tin and it's still performing. I open it, instantly cover it with lint free cloth, tip, and cap back on. Ok, I have to take the cap off and put it back on every 10 seconds if Im doing a large mould, but every second that its exposed to air degenerates the chemical. Its not evaporation you have to worry about as much as moisture ingress, and semi perms absorb moisture quite easily.

Are you heat curing?

That looks like shrinkage/low quality gelcoat (maybe over-catalyzed and/or over-pigmented?)

Really depends how many parts you want to pull from the repaired mould. If its only 1 or a fairly small number ie not a production run then I would repair it. One of the easiest ways is to give the mould a really good cleaning with mould cleaner, key the surface with 80's or similar. Then spray the mould with a good few coats of 2k High Build Primer. 2k is nasty stuff so unless you have ALL the required safety equipment I would get your local body shop do do it. 

Hi