help! which plaster do I need to buy? (salt inhibiting)

[cally7]

Hi, Hope someone here can help? I am getting plastering done tomorrow and it needs to be a salt retardant plaster (chimney breast was having damp patches). I need to buy the materials today or tomorrow (in London) so this is a bit urgent.
What plaster do we need to use? Or is it more a case of buying a salt inhibitor addititve?
Should it be sand/cement or something like Hardwall?
Anywhere this stuff can be bought in London on a Sunday? Or a great shop that can talk me through what we need?
Many thanks for any advice!

[chriseccles2]

Safest bet would be a 1:3 cement/sharp sand render, then
skim with Thistle after a few weeks.
-
If the chimney breast is prone to damp, salts will break
through ordinary bonding coat eventually, leading to
popping, scaling and more work.

Have you looked at the causes of the damp ?
Dodgy chimney flaunching ?
Flashings ?
Rising damp ?

Chris Eccles