[drive surgeon]

tried to get mortar off flags for a customer and used brick acid, did ok, but she phoned up after and said the flags were dry but were i put the acid on was stil wet looking. can the acid stain the flags or can you get away with it, it seems fine on block paving.  i thought the flags may have gone whiter were the acid is on.  i pressurewashed it all off.

[Roger Oakley]

Yep, you can damagestain concrete flags with "Brick Acid".  What % acid did you use?
Got a photo you could send me, I'll try to help.
Seen this so many times, (No disrespect) with DIY merchants doing this and having to sort out the mess.

[drive surgeon]

how come it does not affect block paving then? i have tried it lots of times on blocks and its great. why only flags?

[Roger Oakley]

I can't answer the question scientifically, but I think you'll find it is the way they are made, finish and maybe thickness. Block paving withstands a lot more punishment then concrete slabs, IE being driven on for one, which is what they are designed for.