I am about to start tileing a 300 sqft floor using Self Leveling Cement(SLC), on top of a 20 year old 4″ slabe that resembles the surface of the moon with multiple horzonial cracks. My question is what type of Crack Isolation Membrane should I use? Some people have said to use a pour on before the SLC. Some say a membrane completely covering the space to be tiled after the SLC under the ThinSet. This job is for a close friend (or I wouldn’t have taken it) so I want to do my best to keep it from cracking in 20 yrs. Thanks, DeGrog