So after the polarized thread I am hear to learn again from @mcneese.chad 
I got a 2021 domane and the post keeps slipping making the saddle tilt. The design is now a one piece cup in the seatpost. I’ve cleaned it, put in a lot of fiber grip but to no avail. The saddle keeps tilting during rides. I torqued it up just above 8nm, still no joy. Are there any other suggestions to fix it up? Particular brand of fiber grip?
Sounds like you’ve done all I know to try. I tend to use the Finish Line brand fiber grip the most, but have also used the Park as well with good luck.
I think the max torque (written on the cup) is 11 nm, so try a little tighter. I’ve got a customer with a bike with a similar issue and have resorted to trying the replacement cup kit from Trek - will see if that solves it!
Not sure, next to the bolt it says 6-8nm. I seem to have fixed it though. I decided for one last try.
Took the cup out, found that there was little to no fiber grip left (been pushed out?). I cleaned it thoroughly and then applied three layers of hair spray on both the cup and the post. I remembered it was a good solution for slipping seat posts. I let it sit for an hour to dry and then applied lots of fiber grip. Bolted it up and rode over some potholes down the street. Didn’t move at all (checked with a level, not a mm movement). Will test it on some gravel roads tomorrow but it seems to work well now!
Before I recommend this to others though, can hair spray also have negative effects on the carbon? Heard some ghost stories about it. Can I ask for your opinion once more @mcneese.chad?
Thanks all for the useful info.
Can’t say I know one way or the other on hairspray on carbon. We used it on occasion back in the day, for fitting MTB & BMX grips to metal handlebars (before clamp styles). The only negative back then was that it would slip like crazy again if it ever got wet.
Sure is interesting and I’m trying to understand why. Maybe the spray helped prep the surface and give a bit more ‘tack’ to it for the fiber grip to do a better job between the cup and cone?
Yes it’s makes for a rougher and sticky underground, but I’ve always been amazed on how much it can handle. Seen it working on slipping seat post with 100kg riders. Gonna see how it holds up.
I used cyclon carbon paste and perhaps that isn’t working so well either. Felt like there were little granules in it compared to earlier. The tube is placed on its lid, maybe the granules settle over time making the second half of the paste less grippy 
Ordered some park tool paste now.
@mcneese.chad will know better than me, but anecdotally at least I’ve heard multiple stories of cracked Trek seattubes from having to overtighten the seatclamp - be careful and take it back to the shop or at least a Trek dealer if in doubt.
On my steel bike I had to resort to a small cut strip of aluminium (it’s 0.2mm thick, apparently) from a coke can to get the seatpost to hold. Whether or not such a strategy would work on an aero post, I don’t know.
This is not a dig at Trek at all, but does no one else think on bikes costing thousands stuff like this should ‘just work’?
This and your example of the aluminum shim are not related to this discussion. You are talking about the post install into the frame and slipping down of seat height.
This post is talking about Trek’s single-screw head that holds the saddle. It is a known issue with just about any single-screw design from any maker. I have seen issues with ones from Spec and other brands. This topic covers the ways to get it stable in most cases.
Agreed, and it is likely a reason we see Trek has adopted a double-screw head on most of their recent seat posts and seat masts. So they have worked to address this known issue on at least some models.
My 2020 SLR 7 has this new style clamp arrangement and it is a massive improvement over what my 2019 SL 6 had (which I always hated). It is so nice to have independent adjustments for the seat fore/aft positioning and seat angle. And no slipping or getting stuck like the old style had. They should really adopt the new style to all of their bikes.
Thank you so much @mcneese.chad!!!
Thanks! After reading your reply and others, discovered one cone could rotate while the other cone was frozen. Three gentle taps with flat blade popped it out. New saddle the other day for my 2018 Èmonda. It didn’t feel seated 100% and indeed slipped nose down during two rides.
I think I’ve got it now. . Thanks again!
Good deal, glad to have helped 