Jobst Brandt wrote:
Bill Sornson wrote:

Over the course of an hour' ride my seatpost will get pounded into
the frame. This is a stock Trek 7200 hybrid and I'm reluctant to
tighten the [nylon?] clamp any more. (Yes, I'm a large rider.)

The problem will only get worse over time. I'm considering green
Loctite. Will that work? Anyone have a better suggestion?

Lots of things you can do. Assuming the post is correct (accurate)
size, you can try a little plumber's tape or other "shim type" fix;
or use a clamp of sorts above the insertion point to help keep the
post from sliding down. (Plastic blinky holder or a full on hose
clamp, for examples.)

The OE clamp shouldn't be nylon, I wouldn't think. Mine are all
metal and I tend to over-tighten stuff; haven't had a problem yet.

Loctite might work, but I'd worry about it causing a "frozen" post.
Others may have experience with it though...

If it's really sliding all the way into the frame on rides, then I'd
say your post is undersized. Visit a bike shop to measure, maybe?

Well, plumbers tape (ain't tape) is thick steel.

Geez, Jobst, for a humorless pedant you sure are funny sometimes.

A) It's plumber's tape with an apostrophe. B) The common variety sold in
Home Despot and every other hardware store is thin, teflon wrapping, not
"thick steel" /strapping/.

Very first Google hit: http://www.wisegeek.com/what-is-plumbers-tape.htm

I'd try a section
out of a soda-pop can or beer can. They are easily cut with household
scissors into a one inch wide band, and see if that can be made to
fit. One full wrap or even more should do the job if it really is an
undersized post that it seems to be. Use grease for final tightening
to prevent future water intrusion.

Jobst Brandt

Since it's sliding down so dramatically, you might be right about the fix.
You might have replied to the OP with that, and skipped the needless (and
wrong) nitpick about plumber's tape.

HTH! (But not betting on it.)

BS