Perfect Render mix ratio for retaining wall/ flower bed

Make sure the wall is free of moss or loose mortar, you may need to wet the block before you apply the scratch coat as too much suction will cause the first coat to dry to quick and crack. The science behind cement render is that each coat should be weaker than the previous, I.e. 1 cement to 4 sand for scratch coat then 1 cement to 5 sand for render coat. For your job I would say that a 1 cement to 5 sand is suitable for the scratch coat. And 1 cement to 6 for the render coat. You will not need lime as generally this was used as a plasterciser which has been replaced by Feb. Lime is sometimes used for the render coat in pebble dashing. Put feb in both coats as per manufacturers instructions, this makes the cement more ‘workable’. You would be advised to put waterproof admixture in both coats also. Again as per the manufacturers instructions. Plasterers often put a third of the recommended waterproof dose in the scratch coat, this will reduce the suction for your top coat, too much will make the second coat slip off, then full recommended dose for the top coat. Hope this helps.

Make sure all surfaces are clean and dust free I would advise a slurry coat of sbr and cement painted on before the scratch coat mix ,this will allow for better adhesionfor scratch coat .scracht coast is 1cement 1washed sharpsand and 4 sand either building sand or plastering sand second coat 1cement too 4 building sand or plastering sand .waterpfroofing liquid can also be added.the reason for the sharp sand in the first coat will make the mix extra strong hope this helps