Most people I know who have grown clematis from seed don't nick the seed coat. The easiest way to get seed to germinate is to plant it in the fall, allow it to go through a cold period, and the seeds normally germinate the following spring to summer. Some have germinated for me in the winter. Some clematis, most notably the tanguticas and integrifolias, don't require the cold treatment to get germinations.

I have in the past used just regular potting mix from the store and the % germinations I have gotten have been low--I think most notably due to rot. This fall I am going to experiment with used perlite that is barely moist and see if that increases the % germination. I have also heard of people using straight perlite for germination.

Mike Chicago who posts here off and on has rolled up seed in moist paper towels and placed them in ziplock bags to get the seeds to germinate (the roots will develop first) and then plants the seeds with roots that have developed into pots of potting mix. He also has used a mix of coco fibers, vermiculite and one other ingredient.

Brian Collingwood has an extensive site where he talks about raising clematis from seed, length of time it takes various cultivars to germinate, etc.-sort of a one stop shop for how to grow clematis from seed. I will link it below.

Here is a link that might be useful: Brian Collingwood's site