Had the same problem half a year ago, problem was not using enough hardener in my case and cold temperatures or a drop in temperatures. 

Hi Dokomo,

You seem to have a good handle on this now but for what it's worth, we generally advise a catalyst ratio of 3% now for the Uni-Mould Tooling Gelcoat - this is for the exact reason that you're enquiring about - at 2%, even around 20'C - it can be very slow. When it cures slowly, it exposes the materials in the pattern (in your case the PPG coat) to the styrene in the vinylester for a long time which can start to attack the coating. Likewise, if you double-apply the tooling gelcoat over the top of itself you can potentially have the same problem, if the previous layer is under-cured then the styrene in the second layer can attack the first layer. Running at 3% (in typical 20'C conditions) pretty-much eliminates this problem and results are generally better all round. In cooler conditions, slightly higher catalyst ratios might be necessary but ideally 3% at 20'C ambient will be very reliable.

I hope this helps and look forward to hear how you get on.

Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales