Hi all,small bit of a problem.About a month ago myself and my mate took on to plaster a concrete wall about 12ft high by 20ft long. We maintained that a good polybond scud,scratch coat and finishing coat would be an ideal method of work for the said area. We mixed the scud at 2 sand to 1 cement with polybond mixed through.The concrete wall was dampened before the scudding. We scratched it 2 days later with a scratch coat of 4 sand to 1 cement.Dampened wall prior to plastering with water as normal. 4 days later we applied the finish coat at 4 sand,1 cement,1/2 lime. All went smoothly until I was passing it the other day and saw a few cracks appearing in it. I stopped and investigated more and there is a hollow sound which tells me the render did not key as it should have. I removed a small area and the scratch coat seems not to have keyed to the scudding coat.The scudding coat is "welded" to the concrete but the scratch coat seems to have only been "resting" on the scud.It came off no problem at all.Can someone point me in the right direction as to what we did wrong as it is driving me crazy every time I pass it out.Should we have applied a bonding liquid to the cured scud coat before the scratch coat or.......?we have done other areas in the same manner before and havnt had any problems. Im very interested to hear peoples solutions and reasons