On 15 Apr 2007 11:44:30 -0700, wrote:

Bathroom redo, with ceramic tile.....

Following all the DIY sites & friends' advice, I put down thinset on
my bathroom subfloor, then my backerboard, then I put thinset in the
seams of the backerboard, pressed fiberglass tape down in the seam/
thinset, then put some more thinset over the tape. I thought while
doing it it was pretty level. But now that everything's in place, I
put a test tile down on the seam & it rocks back & forth a bit.

Should I go to the trouble of grinding down the seams to get the seam
perfectly flat? Or will the thinset that goes under the tile handle
this minor hump?

You did it right. The thinset will handle the bump unless it is
really bad. How bad is really bad? Depends on the tile size and
thickness, (and hence, on the trowel notch size and thickness of
thinset). If you're really worried about, lay a couple of test tiles
using the proper notched trowel. You can always pick them up and
scrape off the thinset if you don't like what you see. If it's a
problem, just skim coat all the backerboard with thinset rather than
trying to grind down the seams.

HTH,

Paul