Looking at the best way to damp proof this wall previously having rising damp issues wall has been injected outside but it’s 9inch thick so was told to inject the inside also but was a bit tricky as the wall is just lumps of stone in the inside with next to no mortar. tried a DPM but wall started crumbling so can’t get a good enough fix for the membrane. I’ve seen so many videos saying render it and so many saying no! 5:1? scratch coat 6:1 top coat with SBR in the mix. Does this really work???