Degreaed it thoroughly, using a seat post clamp that has a bolt and not a QR, tried carbon paste, …usually that works but it still slips a bit.
Had the digital calipers out and everything is measuring 27.2 or near as damm it.

Sand the seat tube a bit to ruff up the interface???? but then it will be marginally wider and more prone to slipping??

Try a seat clamp that laps a bit more??

Any componds out there I can use that stop slippage while still making sure the post doesn’t doesn’t get stuck????

How much seatpost is actually in the frame when the seatclamp is done up?

I was having similar issues with one of my bikes, 27.2 seat post and a bolt up clamp like you too, not matter how tight it would always slip… I ended up changing the seat clamp to a Salsa Lip-Lock and have not had a problem since.

I think it’s because the insert that the bolt screws into can turn a little as you do the clamp up, so you can do the clamp up tighter.

Try Hylomar, it’s a non setting gasket compound recommended to me by a guy from Trek USA. Works a treat, a bit like sticky candle wax. Available from all good motor stores…

Had a frickin’ nightmare with my Blue Pig with the post slipping. Bought a 27.3mm post (made by Ragley – hmmmmm wonder why that’d be then…) which was an improvement but didn’t sort it fully, since made up a shim from a beer can and all seems proper snug now. Not had a chance to ride it properly since fitting the beershim, but fingers crossed…..

A new product! try FINISH LINE FIBER GRIP. Grease with grippy stuff in it. Designed for carbon bikes but works on Alu & Steel ones. Stops slippage and corrosion. I bought some for my carbon bike but now use it on the steel & alu ones too