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? 

Painting composite parts that have no in mould coating or gelcoat will often still have surface pin holes to some extent and then also the issues you get with trying to paint a pinhole surface.