Originally Posted by FBinNY

Before spending serious dough making a post which may not solve the problem anyway, try to find out why your post is slipping, and solve the problem at the source.

Thousands of Bromptons have been with the same post and obviously they don't all slip. So what's different with yours? Possibly you can solve the problem with something as simple as cleaning both the post ind inside the frame's clamping area (top 2-3 inches). Otherwise the problem may be a cracked clamp, or ear on the frame, or a bad cam on the QR.

If all is 100% and you still have slippage, possibly because you're heavier than average, here's a cheap fix using the same concept they use to make non-slip stair treads. Buy some carborundum in a medium grit, say 120, and some clear nail polish. You're going to paint a traction band in the 2" clamping area. The beauty of this is that it's a cheap, reversable fix which can save you serious dough.

Start by marking the post above the frame tube when it's at your riding height. Clean the area completely with acetone, or another solvent that evaporates 100% dry. Paint the area with a mix of the paint and grit. There's no one best way to do this, you can dip the wet brush and pick up grit, or you can paint the post and apply the grit into the tacky paint. Once the grit paint is dry and cured (24hrs) you should find you have excellent hold, at a cost of fifteen minutes and a few dollars.