watch these two vids--great stuff

http://www.talkcomposites.com/4746/flanged-mold-construction?Keywords=first%20project

CarbonFiberCreations

Oh ok, I think what I'm gonna try next is fibre putty for the annoying corners and edges, with layers of CSM on top to reinforce the entire mould.

If you are working in a small space then the fumes from the polyester can be overpowering. But from a health respect my understanding is that the "danger" comes from actual skin contact with the resin/catalyst not the fumes. 

Carbon Copies Ltd

Using chopped stand matt with epoxy powder bound.  Emulsion bound for polyester or vynelester,working with powder bound can be hard work make sure you use low viscosity resin.   

Oh that makes sense. Does the uni-mould polyester resin more VOCs? I've been doing this in a shed and letting it cure at home because of the cold.

The Epoxy Resin could be the problem. Depending on which CSM you used epoxy won't break down the binders in the CSM. Therefor it remains stiff and inflexible. The Uni-Mould system is excellent, it's a three part system and works really well. I use it all the time and my moulds come out bang on first time every time.

Carbon Copies Ltd

I didn't use a roller but I probably will next time. I used normal EL2 epoxy laminating resin and ended up using more than I thought I would making it quite expensive. I just realised, should I be using that uni-mould tooling resin? It seems so much cheaper, does this work with the clear epoxy gel coat or do I have to get the uni mould one?

I have had mixed results when trying to repair moulds. If it is being used for wet lay then you have a chance. If you a planning to infuse from it then remember just the smallest pin hole will prevent you from holding vacuum. If the part is a one off then I would say give it a go, but if you want to pull a few parts from it then make a fresh mould it will work out cheaper and less hassle in the long run.

Carbon Copies Ltd

Are you using a finned/bristle roller when you lay up? You need to give the whole area a good going over with the roller especially any corners particularly internal ones. This should get rid of any trapped air. Also after using the roller I go over any difficult areas with the heat gun, this also helps to "pop" any small air bubbles. Hope that helps. Also for moulds I tend to make them resin rich.