Can anyone please advise on the use of salt neutralising solutions for use on good plastered walls, after having a chemical damp proof course injected. Our son is buying a terraced property which has newly plastered walls which read above acceptable levels with a test meter. We could do the damp injection again but it would be a shame to have to hack off the plaster as there aren't any badly blown sections and obviously the cost would be considerably less. Having said that I would not want him to have to replace the plastering in the future because the neutralising solutions were not working.
Coating the wall with PVA will not resolve or neutralise the hygroscopic salts contained within the plaster. I will only seal it in along with any moisture contained within the plaster. Permaguard produce a salt neutralising solution that can be painted on the wall prior to decorating. You can PVA the wall after treatment if preferred although it is not necessary and should only be used if the wall is completely dry as you are locking the moisture in the wall and disabling natural evaporation from the plaster surface.
Please note that if the Chemical DPC has not been installed properly, these salts will continue to contaminate the plaster. Although you do not want to remove the plaster, I would always advise it due to hygroscopic salts absorbing atmospheric moisture on days of high relative humidity and therefore the plaster will never fully dry out.
If you were to re-render/plaster the wall, a salt inhibiting cement should be used at a ratio of 3-1 with a waterproofing chemical such as Sika 1. This will inhibit any salt permeating from ground water as well as salts contained naturally within standard masonry.
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