The reason why wax is better as the top coat (or only release coat) for wet lay up and where gelcoats are used, is that semi-permanents can make the surface so slippery that thick gelcoats end up fish-eying and separating on the surface. So wax "dulls" down the slippery surface a bit making it easier to get the gelcoat on.
As Fasta mentioned it is also good insurance to use 2 release agents like that. We especially recommend it for taking moulds from patterns where you only get one chance. PVA is even safer chance of release but it leaves surface marring meaning you need to polish the mould a fair bit to get a good surface.
Easy Composites / Carbon Mods - Technical Support Assistant