Plastering damp line
We've recently had damp proofing carried out throughout our downstairs. They took 1.2m of plaster off round the bottom half of the walls, back to brick and treated them. Then rendered, plasters etc and went on their way. We waited the advised 3 weeks, sanded down the wall as there was an obvious difference between old and new and just wanted to smooth it all off so it looked less obvious. But now we've painted we have a wet mark all the way round where the old plaster met the new plaster. What have we done wrong/ how can we fix it or is it something that's gone wrong with the plastering?
Advice from 5 local tradespeople
What’s likely happening
When they removed plaster to ~1.2m and treated the walls, they created a new breathable system at the bottom—but the old plaster above is still different in composition and moisture behavior.
That line you’re seeing where old meets new is usually called a “tide mark” or moisture transition line, and it can happen because:
1. Residual moisture is still drying out
Even though you waited 3 weeks, walls that had damp treatment can take weeks to months to fully dry—especially solid brick walls.
If it's showing still wet it needed longer to dry out before sanding and painting
This should not happen, I would suggest you get the company who carries out the damp proofing back in to assess. It’s sound alien it hasn’t been completed correctly.
Number of things,
Damp raises to 1.2, the cut off should be 1.5 to allow extra breathing.
Another is, what’s it been treated with ?!
What are the bricks? Anything pre 1945 could be lime based so, lime may be the answer to helping this.
Locating of where the damp is coming from. Gutters, drains, downpipes, dpc etc. correcting that before making decision on plastering.
Shouldn’t need sanding… minimal if any!
Depends what they used did they sand cement waterproof or have they bonded it which will cause endless problems plus you did not have to sand it it should be blended in nice ready for decoration plus never use vinyl silk on new plaster always emulsion waterd down on first coat but make sure dry first
