Paint is a terribly hard thing to stick to with cement products. In my
experience
of tens of thousands of tiles removed with chisel hammers, paint will
release
thinset faster than anything, so in my experience, it acts the opposite
of a "link".

Real "link" is that blue gooey stuff that is made specifically to link
new cement
to an old cement floor. I don't think it's expensive, and you simply
brush a
coating on and let it get tacky before applying the new cement mix.

Another option would be to make your own polymer thinset by combining
latex additive (made to be added to thinset) with plain thinset. Use
this
same latex liquid as a link by brushing it on straight, then letting it
dry
before you skimcoat with the latex/thinset mix.

You need to be careful to never mix latex additive to thinset that is
already
latex modified in it's dry form. If you use liquid additive, use
plain
thinset (the cheap stuff).

thetiler