I've been offered the use of a split mould to make a clamshell for my car.  Seems rude not to!  However, its a large split mould with no sealing channels in the flange.  It got me thinking....... Can I seal over the split line with a compatible gel coat prior to infusion or is the gel coat stripe likely to let air by? 

A paint primer can spray much thinner than a gelcoat so it saves weight, plus an easy prep for topcoating when all you need is a light wet/dry paper or even just a scotchbrite pad plus appropriate prep-sol or other to ensure there is no contamination issues or fish eyes with your top coat.