Whats the chances of a uni mould being porous? 

Im on my second attempt at trying to get a leak free bag on this car hatch back but still no luck.
I feel I'm no amateur at the bagging process but not having much joy with this one.

you have chances to get leaks, 
But I think its all a matter of how you work on your mould...
If you use all the steps; gelcoat, coupling coat, tooling resin (4 layers) the chances are minimal! 
make sure you have no airpockets in your tooling resin coats by using a roller and tighten all the corners firmly against your part while making the mould. 

If you want to be fully sure; I sand the back of the mould once cured, removing all sharp edges and coat it again with a thick layer of resin.

Matthieu Libeert
Founder MAT2 Composites X Sports
website: www.mat2composites.com