Solid external walls are not suitable for the direct bond system as these areas can be prone to moisture ingress.

Cavity walls are suitable as the cavity minimizes the amount of water that can be pulled through by the dab. 

The bonding compound can draw in moisture from the masonry background which is absorbed by the adhesive compound. This can cause the degradation of the adhesive dab which may lead to failure. 

One alternate option would be to mechanically fix a GTEC Shallow wall channel direct to the background. This will require the background to be in good condition and level/plumb as this method offers no ability to take out any unevenness. 

If the background is uneven the second alternative will be to use the dryliner system which is a separate frame. Both cases will require suitable fixings to the background.