On Thu, 11 Jul 2013 13:19:32 +0100, The Other Mike wrote:

It's failed again, because there must be moss embeded into the
stone/old lime mortar.

Pressure washer?

Nawp, the wall was pressure washed two years ago to remove the old
flakey paint before being repainted. Last year one of the spots was
scraped out and wirebrushed before being repainted. This year the
damn stuff is back and the paint failed...

The moss must be well emeded into the stone/mortar so the treatment
needs to get right down into the roots and kill them.

--
Cheers
Dave.