I have a Gravity Dropper post in an Genesis 853 frame, with a Thompson clamp.

If I have the clamp tight enough to stop the post slipping down, the dropper doesn’t return properly. If I loosen it enough to allow the dropper to work properly, the post slips down the seat tube. Grr.

Is carbon assembly paste the stuff I need to give the seatpost a bit of grip in the seat tube?

Yup.

Hylomar blue non setting gasketing compound. Can be had for £2.99 for pretty much a lifetime supply. Works a treat.

Have used assembly paste even when there isn’t a problem. Clamp barely needs tightening. Excellent stuff

If I bought a tube of it, where else would I use it on the bike? The only other thing I grease before assembly is a very light smear on the steerer before I put the stem on.

Bar/stem.

Right. A tube’s going to last a while, isn’t it…

No need for a whole tube http://www.wiggle.co.uk/ritchey-liquid-torque/

I would see if you can get some off a friend. You need about 20p’s worth out of a £8 tube.

I used it to hold the crank on my city rat bike when the bolt fell out. It’s a fraction of the price of carbon assembly paste, which is its main selling point.

Or if you have any pumps to assemble, you could use it there.