Window AC w/Sealed Room

I am considering a new sealed room grow and I'm wondering what steps need to be taken to ensure that no air escapes around a window AC unit. I understand that the unit needs to have the option to close vents and recirculate only. I know I need to caulk all cracks surrounding the window unit installation. All that is pretty straight forward. What I'm wondering about is the interior seal of the AC. I have heard that this seal can break down over time and allow air (and odor) to escape. What is the best way to ensure an airtight seal on the interior of the AC unit? Also, I have heard about people using black spray paint to eliminate light leaks, but this doesn't make much sense to me, why would black paint on the interior of the AC keep light from escaping? I have seen people use light tight black ducting attached to the front of the AC to block the light leak and also direct the cool air where they want it. I'm pretty sure that is the route I will be taking, but I wouldn't mind hearing if anyone has any insight on the black spray paint bit. I feel pretty confident that I can seal the window AC and keep the smell contained, but I would like to hear any insight I can before attempting this. I cannot afford to have any air exchange to outside.