We do not recommend applying any of our smoothing compounds to ‘black jack’ type damp proof membranes/paints/protective coatings.
These types of membranes are usually based on bitumen but are softer than bitumen adhesive residues which remain after the removal of old flooring.
Due to the softness of ‘black jack’ it does not provide adequate hardness and restraint to support the strength development of a smoothing compound. If a compound were to be applied, it may initially adhere, but as it hardens over time it will most likely fail with debonding occurring within the soft ‘black jack’ layer.
If a product is to be applied to ‘black jack’ type damp proof membranes this would be an unbonded cement and sand screed e.g. ARDEX A29, A35, A38 at 50mm, an unbonded A35 MIX/A38 MIX at 40mm, or an unbonded calcium sulphate screed, A70 at 30/40mm. With all of these unbonded systems the screed mortar does not adhere to the ‘black jack’ but in effect sits on this layer due to its own weight with the above thicknesses being the minimum at which they can be applied.