Nicholas wrote:

Hi

I was told not to use brick acid as it can damage/weaken the mortar holding
the wall together...

Well so it does. But not enough to be an issue, because you pull the big
lumps off first with a chisel or wire brush and use the acid to freshen
up the surface only.

The mortar joints will fizz, but we are talking the merest mm or so
degradation.

Hose the wall afterwards tho.


I'd probably ignore that advice if the wall was really messy though but make
sure you clean off very thoroughly.

Cheers

Nicholas