Hello Mauro,
Firstly, the thinset should be mixed to the right consistency. Different thinsets require different ratios of water, so you'll have to experiment a bit. What you are after is thinset which is creamy enough to spread smoothly but still stiff enough that the ridges hold their edges.
When spreading, use the flat side of your trowel to press the thinset into the surface first to establish a complete bond. You can also use the flat side to roughly distribute your thinset prior to combing. When combing the thinset, you'll want to hold your trowel as near to perpendicular as you can comfortable hold it and press the teeth of the trowel firmly to the ground, ensuring that the teeth are able to gauge the thinset bed. Comb in straight lines, checking that your ridges are consistent and continuous. It takes a little practice to get the right "rough" amount of thinset onto the floor so that the ridges are full but you don't have mountains of thinset you are pushing around.
I hope that helped?
Shaughnn

Exactly as shaughnn described, both the consistency and technique impact what you have described. If your thinset has too much water, your ridges will not hold the edges. Do press the teeth of the notched trowel firmly to the substrate to make certain you have an even amount of thinset under your tiles. I made the mistake with some of my first tiles of not pushing the teeth all the way to the thinset and it uneven amounts of thinset beneath the tiles and made it difficult to get the surface of the tiles even (no lippage).

Mongo,
That's what I said. :^D
Shaughnn

Well, in that case...then what you said I said pricklypear said you said. lol

Ok... Thanks for the advice folks but... At the risk of sounding a little slow... Pricklypearcactus suggested press(ing) the teeth of the notched trowel firmly to the substrate - which is what I was doing - and then I was ending up with plywood between the ridges.

Is it pressing too hard or is it the "burning in" part I'm doing wrong?

(Also, thanks for the 'straight line' tip - it should be easier to gauge thicknesses in straight lines than in the 'arcs' I was )doing.

A lot of people never "burn in" the thinset to the substrate. They'll just plop it down and pull it out with the notched side. Doing that, you'll often get really clean substrate in the valleys. Which means that underneath the ridges, you're really not getting the best bond between the thinset and the substrate. The thinset is sort of just sitting on top of the substrate.

So if you burn it in, then use the notched edge to rake it out, yes, you'll see cement board (or whatever your substrate is) in the valleys. But it'll be schmeared with a bit of thinset.

Then when you place the tile and press it into place, the thinset ridges get knocked down and the thinset fills the valleys, and the thinset that fills the valleys will have a better bond with the substrate since the substrate has that schmear of thinset burned in to it.

So yes, burn in the thinset with the flat side of the trowel, then rake it out with the notched side, pressing the teeth against the substrate as you rake it out. You're not looking to use so much pressure that you "dig" into the substrate...you're just sort of lightly "scraping" the surface of the substrate with the teeth of the trowel.

If using cement board, you'll see the surface of the cement board, but the pores of the cement board will be filled with thinset.

That's my story (and shaughnn's, and pricklypears's) and I'm sticking with it. ; )

All that talk about schmears, I feel the need for a bagel!

Best, Mongo

hi Msavold,
It *used* to be the industry recommendation that thinset be applied in pretty arcs. The thinking was that these arcs crossed through any linear joint lines and so gave the tile full support instead of easily compressing if a tile's edge happened to align with a combing void. We *now* know that combing thinset in straight lines allows air to escape from under the tile when it's being installed and so the bond is improved and we have less incidence of failure.
The arcs were the standard for a long long while but we're constantly questioning our process and adjusting our practices as our understanding improves with technology.
:^)
Shaughnn

Aha!
Thank you gentlemen!

Mongo: you reassured me! Turns out I was *almost* doing it right - I was under the impression that I wasn't supposed to see *any* substrate - heck, the Ditra sketches even show a measurable thickness of thinset in the valleys - about 1/3 the height of the ridge. And because they certainly used Photoshop (:p) it's a *perfect* uniform thickness too! Me, I was seeing blotches of plywood or orange Ditra... and I was worried. The 'almost' part is mostly the consistency. I think I was mixing it much too dry.

Shaugnn: makes perfect sense - about the air escaping. It's amazing what sort of a vacuum the tile/thinset generates when you try to pull a mislaid one up! And the bonus is that the lines do seem easier.

Thanks again for the wisdom!
Now, back to tiling (or bagels, Mongo ;)