Don't want to come across as a **** for linking to a competitor (they are a whole quid more expensive anyway!)

However, they do tell you which nozzle is recommended for what:

http://www.ecfibreglasssupplies.co.uk/p-1698-g100-spray-gun.aspx

Hi, just bought your gelcoat spraying gun. Im planning on using it with Axson GC1 050 + GC10 epoxy gelcoat which has rather high viscosity. The standard nozzle size didn't work at all. What size would you recommend me? How big can I drill the standard nozzle?