Quote:
Originally Posted by rab60bit
You're right to first suspect the substrate.
Most 'old' slabs need to be physically or chemically scoured and then sealed before painting (and good paint is nearly always two-part chemical reactive). New slabs usually form some sort of calcium/salt surface layer as they dry out/cure and that too needs to be removed/neutralised before any painting. Not sure about how 'dry' it needs to be but if there isn't a damp proof membrane (DPM) then dampness will always migrate to the surface in which case you'll need something special (£££) so speak to the experts.
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looks and feels dry so do not think damp or dirt to be the issue, just poor grade, mixing, ratio of concrete.
macafee2