Why does the salts come out on some gypsum plaster (that white crystal stuff)?
Question:I do a bit of plastering and some times I get the salts come through and blow off any paint, Are they painting too soon ? though it only comes up "here & there" and then it happens on un-painted as well, is it damp in the walls? How do you best get rid of it and prevent it coming back?
Answers:
Hi,
I'm a plasterer and have come across this a few times and been asked to repair it. The cause could be several things, the most common being damp. Other less common causes could be unwashed sand being used in the original backing render. The way I deal with it is as follows. First of all, take the contaminated plaster and backing coat off (down to the brickwork). I then get my local damp preservation guy to check the brickwork for damp. If it's damp, it needs to be treated professionally. After treatment you have to wait for any damp to escape from the brickwork (usually around 3 months!) After this period, brush off any remaining signs of salt and use a salt inhibitor (usually in liquid form and brushed on). You can now start to apply the backing coat. I use either a renovating backing coat or "Thistle dry coat" Once this has been applied, you can finish off using Thistle multi finish the day after. When all is fully dry, it can be painted using a matt emulsion watered down to about 10%. If the problem is just contaminated backing coat and not damp, do exactly the same as above but without the damp preservation process.
Hope this helps...
Evanescence, it the problem and an acid wash plus when dry, a good primer to lock in the reaction. Block filler and two topcoats should slow the problem down. Moisture from the outside may be hard to control and in older structures that is hard to do. To totally fix the problem the exterior concrete must be sealed. Dig down and put a moisture barrier in place.
Its coming through from the bricks and not the plaster. Thats why you are not seeing it everytime you plaster.
The salts are drawn out of the brick by damp so you need to cure the damp BEFORE you plaster. try bitamen sealer on the brick,chemical injection, or using lime-based paint.
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