The cement should be added to the SBR and continually stirred until its the consistancy of porridge. This is usually 1 part SBR to 5 parts cement. Its not a great idea to put it on top of render as, when a wall is damp, its usually the render that soaks the water up vertically. This means that there is a likelyhood of the render still being damp and this could well push the render off the wall together with your new coating. To be totally effective the new tanking should be applied to the original surface of the wall with the render taken off and any holes filled with a mkix of sharp sand and cement at 3 to 1.
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