Weird white fur/fluff appearing on new plaster - what is it?

[Nick]

Hi,

Got up bright and early this morning to find some fur/fluff on the surface of some plastering that was done earlier in the week for a hole in the wall fire. I've no idea what it is (hopefully not mould!), and would welcome any advice as I've never seen this on any other plastering.

There was already a large opening for an existing fireplace, so that was bricked up where necessary and the brickwork plastered. The plaster on the new brickwork has lightened in colour as it should and seems fine. Some of the existing brickwork around the hole was also exposed and needed to be filled, and it's the plaster onto the existing brickwork that hasn't lightened in colour in the same way and that now has the white fluff. The depth from wall surface to existing brickwork was a good 10 to 20 mm at least, and I'm not sure whether the fitter used plasterboard to pack out the bulk of this depth before plastering or whether it's filled totally with plaster. I suspect the latter, and given the depth, I could imagine that this may not have dried yet hence not going light in colour.

Any ideas on what the fluff is are most welcome, and also whether anything needs to be done about it other than getting it off somehow.

Cheers Nick

[diypaint]

do a google search using the words "Efflorescence" and "plaster" and see what you get....

is the most likely explaination. cheers

[The Wanderer]

Possibly alkaline salts. You mentioned that the prob seems to be where older bricks have been exposed and plastered over. It might be that the damp of the plaster has leached these out of the older masonry.

Personally, I'd give the plaster a first coat of an alkali resistant primer.

[Nick]

Many thanks for the quick and helpful replies and the right keyword. That seems to be exactly what the issue is and no major problem.

[The Natural Philosopher]

Its salts being carried from the wet brickwork to the surface by capillary action.

It will stop once (if?) the underlying brickwork dries put completely.

If it doesn't stop, you have a damp problem.