It's BETTER to leave it overnight. Last thing you want is the styrene to attack your gelcoat and cause alligatoring. It may be cured enough after a few hours, but it's a risk. 

In the PDF instructions it says to leave the initial gel coat overnight to cure before the coupling coat is applied but in the video for the car bonnet part 1 using Unimold it states to wait a few hours. Which is correct?

check out the tutorial video on making the car bonnet... they go through the Uni-mould system quite thoroughly in it. (Part 2)

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Do not adjust our mind, theres a fault in reality :-)

I just got the Unimold starter kit - I have read the guide showing the process but am a bit unsure of the process between the coupling coat and the final reinforcement layup. Before laying up the final reinforcement Do you 1. Allow the coupling coat to completely cure. 2. Allow the coupling coat to tack, or 3. Apply the final layers immediately over the wet coupling layers.