Render Layer Mask Problem

[MoGrafik]

Hi everyone,
I was hoping someone could please help me figure out layer masks. I've done the tutorials infinitethrill kindly posted up, but it's not working for me.
I'm looking to render masks / luma mattes of scene objects for compositing. I've added octane object tags to the objects I want masks for, ticket 'object Is' and given them a number, and ticked 'render layer mask' in the render settings. However, octane renders the mattes after the main render (even if I untick start after beauty), and takes surprisingly long, considering it's just generating a black and white matte. Also, when I bring in the matte in After Effects, it has very blurry / noisy edges, regardless of how high the samples are. Is there something I'm doing wrong here?
Thanks for the help