Confused.com We've just finished major renovation and house needs rendering due to new steels, windows/doors etc. House already rendered with colour in it and painted over as well. Various companies that supply render products are saying you cannot render over existing render/paint but to (A) take old render off (B) sand blast paint off (c) fix insulation boards otherwise no warranty. BUT getting mixed message from those that actually do! Some of the render has blown and there are a few cracks. Being told blown render will be removed and then exisiting render will be bonded and then scratch rendered / mesh / top coat etc. applied.
Are you a tradesperson and able to answer this question?
Rendering over existing render which has been painted is never a good idea due to a few reasons. The main concerns are that the weight of the new render is simply hanging onto a layer of paint and the existing render with very little key and so if the paint or existing render was to delaminate then your new render will go with it. There is however a couple of options as you have said with using insulation over your existing render, removing the existing completely or another acceptable method is to mechanically fix your new render into your substrate. You will still need to remove the blown areas of render and any loose paint and the dub them back out however you could then use a render only system over the top with an embedded layer of reinforcing alkali resistant mesh and mechanically fix this layer back into your substrate (brick or blockwork behind all layers of render) with hammer dowel type fixings which will prevent it from going anywhere in the eventuality of any layers becoming loose in the future. This is commonly known as scrim and pin and is another option for you to consider if removing all of the existing render is not an option.
I agree with everything said, another way is polymer render using a red grip product over existing painted area then apply polymer render with a mesh in it which goes on thin.
I agree with the guy first ans , I have seen people rendering over painted walls with the same result!! bad bad bad !! , don"t do it remove whats there first .This way you don"t have issues a few years down the line .
dpf
Answered16 March 2020
0
Anonymous user
If u get rid of the blown Render first. After which an sbr slurry over the painted areas and whilst still wet apply a 3:1 sand and cement mix incorporating more sbr to form a scratch coat. Next day top it. Simple.I done one in arundel 20 years ago. Still solid.
