Sorry I didn't give a larger picture of what I'm doing in the first
post on this topic. This entire apartment has a
concrete floor. We're expanding the kitchen, which was originally
hexagonal mosaic set in about 1-1/4" of
mortar (over the concrete, which is the same level throughout the
place) Over that, they put down 12 X 12 ceramic tiles. We're going to
match these tiles in the expanded area for one continuous floor, so
that means going
down to the concrete where the herringbone was. That means two layers
of concrete board set in mortar and
whatever else to level up, and that's what's got to stick to the black
stuff. A scraper gets down to the level
of the concrete easily, so I guess the question is, will the latex
mortar stick to it? I'll experiment. The Bosch
hammer would be a great acquisition, but pricey. I see at Menards
there's a Bosch 1-1/8" hammer for around $279 but I wonder if it's up
to the job, because I'll be getting other tile jobs here and removing
old tile and concrete
board can be draining. It's hard to stop talking about tools - just
picked up a Rigid Fuego mini sawzall which
fills a gap in the tool market that's been there ever since the Porter
Cable Tiger Cub disappeared about 15
years ago. The Fuego seems to be a quality tool.

If I were going over an old floor with two layers of concrete underlay
sheeting, I wouldn't be that careful about what I was trying to adhere
to as the first layer.

I would certainly clean off all loose and removable debris and
coatings. Some of the latex mastics are quite good, but I would still
go with a solvent based adhesive.

The trick to a good, solid substrate when you have two layers is to
adhere the first layer to the concrete, and the second layer to the
first with plenty of mastic, and a few Tapcon style anchors.

You can make an inch thick solid (near monolithic) subfloor by lapping
your joints properly, adhering properly, and adding a few mechanical
fastners. Installed in this manner, adherence to the concrete is
almost a moot point.

As a matter of fact, in high movement areas of town, when an inch or
so of substrate (board or thickset) is applied under tile, many here
are starting to use a slip sheet method of installation over the
concrete and don't attach the built up material to the concrete at
all. This requires a solid, bondable substrate, but not solid
attachment of same.

I ran into this method about 20 years ago in Houston when remodeling a
house there, and the tile guy I liked put down a sheet of 6 mil, then
built a 1 1/2" mud bed over the plastic. The floor never cracked or
moved where you could see it. Two neighbors remodeling at the same
time were informed that "it wasn't necessary" to put a slip sheet in,
and their tile cracked within a year. The cracking was blamed on slab
movement ( no... really?) and there was no way to hold the tile
contractors directly to blame as he put in the tile as specified in
their respective contracts.

I was last inside that house about 5 years ago, and no cracks in the
floor, even after all these years. That method pretty well accepted
around here now.

As far as tools go, if you see a large tool that would help you zip
through a job, why not rent? We have a lot of rental places around
here that cater to contractors, and they have all kinds of tools. We
even have two Lowe's stores and two HDs that rent tools these days.
And they carry all bits, abrasives, cutters, etc. for the machines
right there in the store, and they don't gouge you when you need to
buy them.

As always, just my 0.02.

Robert