It should have been simple. Clean carbon frame and seatpost thoroughly. Apply carbon paste (Tacx in my case). Tighten to spec (5 Nm). Observe seatpost never slipping.
Alas, it has not been. On longer rides, after 2 hours or so, my seatpost has started slipping, slowly, just a mm at a time, so I hardly noticed it when it first occurred.
I am not sure why. Maybe because my Scott Addict R1 frame is ever so slightly oversized, closer to 31.7mm and the seatpost, Deda Superzero, a tiny bit undersized. This oversize/undersize combination causes the seatpost clamping area to become too narrow. This combined with my long legs far back seat position, and flexing carbon, has caused the slippage. I am not sure though, opinions welcome...
I tried to re-clean, and re-apply more carbon paste. I tried very thin tape to increase the seatpost diameter. No difference. I think the tape eventually got crushed and was of no use.
Now I may have figured out a solution.
First I cleaned yet again, applied a thin layer of acrylic clear nail polish to fill in scratches from the slippage and increase seatpost diameter slightly. Then I carefully sanded the nail polish layer directionally around the post, to make it less slippery. Since I am sanding only the nail polish, the carbon surface is not affected.
Finally I mixed carbon paste with some non-hardening gasket sealant
(from auto shop) and applied a liberal layer of this to seatpost.
And now after several +3 hour rides, the problem appears to be solved...
There's probably a simpler solution, carbon paste only should have worked, and does it most cases.