Pete Verdon coughed up some electrons that declared:

Hello again,

I've been exchanging emails with Mapei about how best to fix my tiling.
Part of the wall in my bathroom is painted; when I asked about this the
Man From Mapei said:

"The tiles will only be bonded as strongly as the paint is fixed so it
might be a good idea to scarify the paint off and apply primer G before
fixing tiling."

That's fundamentally true. How do you know though that the finishing plaster
under the paint isn't applied directly over some ancient paint? See where
I'm going?...

This is as far as I know normal emulsion wall paint (could possibly be a
slightly water-resistant bathroom type). I've never removed wall paint
before; I guess it would be nigh-on impossible if you wanted a finish
suitable for painting over, but as something to take tile adhesive I can
get a bit more agressive. Any suggestions for how to quickly remove it?

Pete

I wouldn't worry about it unless the paint is obviously loose.
Mapei have 2 sorts of tech support:

Helpful, eg: "Well, if you must, use X - we're not guaranteeing it for 20-30
years, but the magic crap in it will give you the best chance of all of our
products in your awkward situation. It'll probably be fine..."

Arse-covering: "You must scrape off every loose molecule otherwise your
entire house will explode!"

BAL Greenstar says to brush and SBR prime polished new plaster. I've never
seen anyone go to those lengths before. Though the SBR makes some sense on
gypsum in a wet room - though again, considering people tile direct to non
water proof plasterboard in showers, it's being a bit pedantic...

How is your paint?

I'd take a scraper to it, or a steel wire brush and if it falls off, it
deserved to and if it sticks then it's probably good enough... I've had
both types in my house...

Cheers

Tim